I re-started to study English

Not necessarily incorrect, but when I first read this I interpreted it as you restarted your entire life in order to learn English (the "to" kind of acts as a shortened version of "in order to". "I restarted learning/studying English" would say that you started doing it again.
Restarted doesn't need a hyphen in between.

I lived in the U.S. for almost 8 years and also in the Philippines for 4 and a half years.

When using fractions smaller than a whole piece (like "half", "quarter", "third", etc.), you would say "and a...", essentially meaning that the "a" is an important addition.

I even studied 13 hours a day at least 3 days a week, and my favorite time was to reward myself for studying for a long time with an hour of online lessons that wasere usually a bit out of my budget, or with something like an English dictionary or book on phrasal verbs/idiomatic expressions.

The sentence felt very long and didn't really have any breaks, so I put in a few commas where possible.
To me "an hour of online lesson" doesn't really sound right - I'd probably either make lessons plural, like I did above, or say "an hour long online lesson", depending on if it was more of a plural or singular lesson situation.

I would listen to podcasts or the radio whenever I walked, eate, driove, or take aook public transportation, or. Otherwise I would just think to myself or talk to myself all in English whenever I had nothing to listen to.

"Podcast" and "radio" both need some kind of article in front, or with podcast you could also make it plural. For podcast, you would use "a" if you kept it singular, and there's no need for any kind of article in front when it's in its plural form, although you could say "some". "The" wouldn't sound right since you haven't referenced any specific podcasts yet, or even properly mentioned any.
I WOULD... whenever, would (in this case at least) = past tense (walk --> walked, eat --> ate, drive --> drove, take --> took)
You would say "to take public transport", and "to take" would change to some other variation depending on the context.
The sentence was getting a little long, and the ideas explored weren't exactly the same, so I decided to split the sentence into two smaller ones.

I would wake up in the morning and opening up a notebook to review what I learned the day before, and I would go to sleep (or fall asleep) while studying English.

"opening" made it sound like you literally woke up by opening the notebook, or in other words, that opening the notebook is the reason why you were awake. "and open" shows that you woke up, THEN you opened the notebook.

I would always make sure that I willould/did not waste even a second doing anything that is irrelevant to my study.

Very small nuances, "did" emphasises the point of it having been in the past though.

When I was in the U.S., I was working with my boss and a few of my colleagues, so we were able to share the burden of our workload we had to accomplish.

"we had to accomplish" is not entirely necessary, but it's ok to leave it in.

But in the Philippines, I was all by myself and had to fend for myself with everything from work to cooking meals three times a day for the entire period.

"period" might sound better if it was replaced with a different word.

The joy to earn a tangible achievement in my work outweighed the joy tof studying English that, which doesn't always pay off as immediately.

Just a few minor things. "joy to study" feels like it might put more emphasis on the studying part, while "joy of studying" focuses more on the actual joy.

One of the biggest reasons I started to see less meaning in immersing myself in studying English is the fact that about 70-80% of the population can speak English as it is their second language, and most of them speak English that is easy to understand even when I go to the bank, dentist, insurance company, and so on.

"as their second language" is more if its guaranteed that it is their second language, while "as it is their second language" puts more emphasis on the actual people who can speak English.

I hate myself for just resting ion my laurels from studying hard in the past and not having as much fluency as I had back then.

Adds a little more clarity and corrects grammar a little.

I lived in the U.S. for almost 8eight years and also in the Philippines for 4 and half1/2 years.

The 8-year period of my life in the U.S. was the culmination of my English study, and I can even bewas proud of myself for staying laser-focused for such a long time.

I just want to use, recall, and cement them in my brain ^^) ¶
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I studied English for at least 8-10 hours every day, no matter how busy I was.

I even studied 13 hours a day, at least 3three days a week, and m. My favorite time wasy to reward myself for studying for a long time was with an hour of online lesson that was usually a bit out of my budget or, with something like an English dictionary, or a book on phrasal verbs/idiomatic expressions.

I would listen to a podcast or the radio whenever I walked, eate, driove, or take aook public transportation, o. Or I would just think to myself or talk to myself all in English whenever I had nothing to listen to.

I would wake up in the morning and opening up a notebook to review what I had learned the day before, and I would go to sleep (or fall asleep) while studying English.

I would always make sure that I willdid not waste even a second doing anything that iwas irrelevant to my study.

When I was in the U.S., I was working with my boss and a few of my colleagues, so we were able to share the burden of our workload we had to accomplish.

But in the Philippines, I was all by myselfalone and had to fend for myself with everything, from work to cooking meals three times a day, for the entire periodtime.

My priority has shifted from improving my English to excelling in my work.

The joy tof earning a tangible achievement in my work outweighed the joy tof studying English that does, which didn't always pay off as immediately.

One of the biggest reasons I started to see less meaning in immersing myself in studying English is the fact that about 70-80% of the population can speak English as it is their second language, and m. Most of them speak English that, which is easy to understand even when I go to the bank, dentist, insurance company, and so on.

Of course, I still loved to study English, but my motivation had surely dwindled.

The opportunity to speak English is almost nonexistent in my current life, and I think that became a huge reality check for me.

I hate myself for just resting ion my laurels from, as opposed to studying hard as I did in the past and not having as much fluency as I had back then.

Dwelling in the past is just doing more harm than good.

I came to the conclusion that I want to go back in time to when I was burning with passion, motivation, and curiosity about learning English.

I re-started to studying English

The 8-year period of my life in the U.S. was the culmination of my English study and I can even be proud of myself for staying laser-focused for such a long time.

(please note that I might sprinkle some vocab or expressions across the entire post that I want to use for my practice, so please feel free to ignore any minor mistakes even if you find it a bit out of context.)

I even studied 13 hours a day at least 3 days a week, and my favorite past-time was to reward myself for studying for a long time with an hour of online lesson that was usually a bit out of my budget or with something like an English dictionary or book on phrasal verbs/idiomatic expressions.

I would listen to podcasts or the radio whenever I walked, eate, driove, or take aook public transportation, or I would just think to myself or talk to myself all in English whenever I had nothing to listen to.

I would wake up in the morning and opening up a notebook to review what I learned the day before, and I would go to sleep (or fall asleep) while studying English.

I would always make sure that I willdid not waste even a second doing anything that iwas irrelevant to my study.

Of course, I still loved to studying English, but my motivation had surely dwindled.

I hate myself just resting ion my laurels from studying hard in the past and not having as much fluency as I had back then.

I re-started tostudying English again / I resumed studying English

You might find this discussion interesting:
https://forum.wordreference.com/threads/restart-gerund.4064967/

I re-started to studying English

"restart" is a "real" word, so you don't need the hyphen. It's only used when you're "making up" a "re-" word, like "re-vacuum" to say you're vacuuming something that is already vacuumed.

I lived in the U.S. for almost 8 years and also in the Philippines for 4 and a half years.

The 8-year period of my life in the U.S. was the culmination of my English study and I can even beam proud of myself for staying laser-focused for such a long time.

"can even be" is grammatically correct, but contextually isn't something I would really use. "even" in this context is something you might use to describe a small or unusual achievement after listing other ones.

(please note that I might sprinkle some vocab or expressions across the entire post that I want to use for my practice, so please feel free to ignore any minor mistakes even if you find it a bit out of context.

the first "please" is enough :D

I even studied 13 hours a day at least 3 days a week, and my favorite time was to reward myself for studying for a long time with an hour of online lessons that was usually a bit out of my budget or with something like an English dictionary or book on phrasal verbs/idiomatic expressions.

or "an hour of an online lesson", but I think it's more natural to just say this -- even if it's only one session.

I would listen to podcasts or the radio whenever I walked, eate, driove, or take aook public transportation, or I would just think to myself or talk to myself all in English whenever I had nothing to listen to.

I would wake up in the morning and opening up a notebook to review what I learned the day before, and I would go to sleep (or fall asleep) while studying English.

"go to sleep (or fall asleep)" very good 👍

I would always make sure that I willould not waste even a second doing anything that is irrelevant to my study goals.

When I was in the U.S., I was working with my boss and a few of my colleagues, so we were able to share the burden of our workload we had to accomplish.

"we had to accomplish" is fine grammatically, but doesn't really add any new information here. It is implied that "our workload" is something "we had to accomplish".

But in the Philippines, I was all by myself and, so had to fend for myself with everything from work to cooking meals three times a day for the entire perduratiodn.

"and" is fine, but ", so" is just better at connecting these two ideas together.

"period" is fine, but, to me, usually implies repetition or structure. "duration" feels more natural here.

My priority hasd shifted from improving my English to excelling in my work.

The joy to earn a tangible achievement in mythrough work outweighed the joy to study English that doesn't always pay off as immediately.

again, "in my" is fine, but "through" is more natural

One of the biggest reasons I started to see less meaning in immersing myself in studying English is the fact that about 70-80% of the population can speak English as it is commonly their second language, and most of them speak English that is easy to understand even when I go to the bank, dentist, insurance company, and so on.

or "as their", depending on how authoritative you want to be about English being specifically their 2nd language.

The opportunity to speak English is almost nonexistent in my current life, and I think that became a huge reality check for me.

I hate myself for just resting ion my laurels from studying hard in the past and for not having as much fluency as I had back then.

the "for" helps to separate these into two separate ideas. Rather than "I hate myself for X&Y" it is "I hate myself for X and I hate myself for Y."

I have to face the reality and accept that I AMam doing nothingNOTHING but just slacking off.

You're emphasizing the "AM" here on purpose, I see, which is cool. But this kind of emphasis would usually be a direct contrast to something explicitly said previously, so it's not as impactful. If you had said something like "I didn't consider it slacking off because I was so busy with other things." and then later said this sentence with the emphasis on "AM" it will hit the reader/listener harder.

Dwelling in the past is just doing more harm than good.

Grammatically correct, but this doesn't feel like the correct use of "dwelling in the past". Typically this would be used for someone who is overly reminiscent or thinking about the past too much (this sounds like almost the opposite situation, where you want to return to how you were before).