A lLate nNight with Insomnia

English uses a "title case." Basically, capitalize the first word, the last word, and any important words in between. Words like "the," "and," "when," or "with" are not important enough to capitalize if they are not the first or last word.

The surroundings was toere so still that the only sound I could hear was the ticking clock on the wall.

If the noun is plural, use "are" or "were." "Surroundings" is a plural noun.

The moment felt so strange and surreal, like I amwas the only person alive in this world.

Verb tenses should remain constant unless there's a good reason to change them. You could also say it this way and add an element of self-dialogue:

The moment felt so strange and surreal, like "Am I the only person alive in this world?"

My heart felt empty ,and yet overwhelmed.

"and yet overwhelmed" isn't a complete thought, so it's better left connected to empty. You could also say:

My heart felt empty, yet it was overwhelmed.

because "it was overwhelmed" is a complete thought.

My tired eyes were wide awakedopen.

Awake is like a mental state, not a physical thing that eyes do.

Although I wished I could fall asleep promptly, my brain was toso relentless that a peaceful rest was the last thing it would do.

"Too" and "so" are related. You could also say:

...my brain was too relentless for a peaceful rest.

or

...my brain was too relentless to rest peacefully.

I checked my phone,. It was already 1AM - a time thatwhen I would be dead asleep on typical days.

For the flow of the story, separate thoughts should sometimes be separate sentences. When talking about a specific time, "when" is sometimes going to make more sense.

Staring at the plain ceiling, I questioned myself, and the purpose of my life.

You questioned 2 things that should be written without separation by punctuation. It's like:

I ate bananas, and oranges.

should be

I ate bananas and oranges.

I would also ruminate on the vague memories of my childhood: --at which point did I started to deviated from my happy, cheerful self?

Two or more adjectives strung together are usually separated by a comma.

I hated it when I experienced an existential crisis because the reality seemed more frightening to face.

verb agreement in tense

At last, I slapped myself hard on the face to shut up those incessant internal monologue flooding inside my head.

A flood just floods something, doesn't flood "in" something. You could also say:

...monologue that caused flooding inside my head.

or

...monologue that flooded my head.

or

...monologue that was flooding my head.

I am just a human being like any others; I shouldn't be so hard on myself like this."

Since you're saying I am a human being like any other human being, and being is singular, other should be singular.

I grabbed my favorite mystery novel from under my pillow, and flipped to the page where I left off.

Without from, it sounds more like you just held it under your pillow.

"The page" is a location, so we should say "where" or "on which" I left off.

A late night with Iinsomnia

This is to maintain consistent capitalization. You can also capitalize the whole title (minus prepositions) like this: "A Late Night with Insomnia."

The surroundings was toere so still that the only sound I could hear was the ticking clock on the wall.

The moment felt so strange and surreal, like I amas if I were the only person alive in this world.

Laying on the bed, I stared at the ceiling after closingturning off my phone.

My heart felt empty ,and yet overwhelmed.

My tired eyes were wide awaked.

Although I wished I could fall asleep promptly, my brain was toso relentless that a peaceful rest was the last thing it wI could dohave.

I checked my phone,. It was already 1 AM - a time that I would be dead asleep at on typical days.

to be asleep "at" 1 AM

Staring at the plain ceiling, I questioned myself, and the purpose of my life.

I would also ruminate on the vague memories of my childhood: at whichat point did I started deviateding from my happy, cheerful self?

I hate it when I experienced an existential crisis because the reality seemeds more frightening to face.

"I hate it" suggests a general hate.
If you only hated the existential crisis in the moment, you could say, "I hated experiencing an existential crisis because reality seemed more frightening to face."

My brain would reflect the most embarrassing moments in my life, and those awkward moments whichthat I wished never existed— it was driving drove me insane.

At last, I slapped myself hard on the face to shut up those incessant internal monologue flooding inside my head.

Monologue is singular

This iwas better!

Under the faint night lamp, the words were a little hard to read, but I could guess themwhat they were.

The surroundings wasere too still that the only sound I could hear was the ticking clock on the wall.

The moment felt so strange and surreal, like I amwas the only person alive in this world.

Laying on the bed, I stared at the ceiling after closingswitching off my phone.

My heart felt empty , and yet overwhelmed.

I checked my phone, Iit was already 1AMam - a time that I would be dead asleep on typical days.

I would also ruminate on the vague memories of my childhood: at which point I started deviateding from my happy cheerful self?

I hate it when I experienced an existential crisis because the reality seemed more frightening to face.

I am just a human being like any others; I shouldn't be so hard on myself like this."

I grabbed my favorite mystery novel under my pillow, and flipped to the page I left off at.