It was something that I had always wishanted to do sincefrom when I was in my early teens.

Not that I was the kind of child who excelswas good at sports, - quite the opposite.

As a child, I remember that I was so bad at playing football, (or soccer as it’s called in some countries,) that I was always chosen to be the goalkeeper.

In my defence, I musthave to say that at least I wasn’t a bad goalkeeper.

“Have to” has a more easygoing, casual feel than “must”.

Not only was I a hopeless case at playing football, but also at playing any team sports.

At the school I went to studyattended for vocational training, I had the opportunity to dotry judo.,

The truth isand it was there that I became very interested in everything related to martial arts.

I’ve connected this sentence to the previous one to make it read more smoothly.

However, I was as good at doingno better at judo asthan I was at playing football.

To make things worse, I was the tallest, which doesidn’t mean I was the strongest, judoka at my gym.

ThiHaving longer limbs might have been an advantage, having longer limbs, but in my case it was a drawback.

Although I was just a yellow belt, and a very bad one, the brown and black belts always picked me out to fight against themto be their opponent.

Earning money allowed me to enrol at a gym where I could do what I had always been wishing twanted to do since I was a teenager.

After so many years of not standing out in any sport, I had finally, I found out that I was really good at lifting weights.my metier

My body responded very fast and my appearance changed radically; from being a thin teenager, I became a very muscular person.

I don’t like to show off.,¶

Hhowever I can’t help but saying that I was the envy of the gymother gym-goers.

I have combined this sentence with the previous one

Sadly, all of this stopsped happening long ago.

After everything I have written, it’s quite easy toSo you can imagine whathow I felt like when a few days ago my older son asked me to go with him to his gym.

I had told him that I thought the exercises he was doing weren’t suitable for him and that he should do another ones.

“Another” is singular, “others” is plural.

At the gym, being surrounded by people lifting weights and especially helping my son improve his training made me remember those past days when I used to go to gymmy gym-going days.

Not to say that I was the kind of child who excelsled at sports, quite the opposite.

As a child, I remember that I was so bad at playing football, (or soccer as it’s called in some countries,) that I was always chosen to be the goalkeeper.

Easier to read with parentheses.

In my defence I must say that at least, I wasn’t a bad goalkeeperat it.

Replaced goalkeeper with "at it" to avoid repeating it in a consecutive sentence. This is just a stylistic suggestion!

Not onlyI was I a hopeless case not only at playing football, but also at playing any team sports.

At the school I went to studyfor vocational training, I had the opportunity to do judo.

The truth is that I became very interested in everything related to martial arts.

To make things/matters worse, I was the tallest, whichbut that doesn’t mean I was the strongest, judoka at my gym.

ThiHaving longer limbs might have been an advantage, having longer limbs, but in my case, it was a drawback.

Although I was just a yellow belt, and a very bad one at that, the brown and black belts always picksingled me out to fight against them.

Earning money allowed to enroll at a gym where I could do what I had always been wishing twanted to do since I was a teenager.

After so many years of not standing out in any sport, I had finally, I found out that I was really good at lifting weights.

My body responded very fast, and my appearance changed radically; from being. I went from a thin teenager, I became to a very muscular person.

Sadly, all of this stopsped happening long ago.

At the gym, being surrounded by people lifting weights and especially helping my son improve his training made me remember those past days when I used to go tomy days back at the gym.

Memories from the Past

Memories = past events
Thus no need to clarify :]

In my twenties and early thirties, I would typically go to the gym almost every day.

Cohesion ! (Implies habitual)

It was somethingan activity that I had always wished to do since I was in my early teens.

This makes it sound a bit more natural! “My early teens” we like to possess it, similar to “my twenties” as you said before

Not that I was the kind of child who excelsled at sports, quite the opposite.

Would be past (excelled) because you are talking about yourself as a child in the past !

As a child, I remember that I was so bad at playing football, or soccer as it’s called in some countries, and that I was always chosen to be the goalkeeper.

In my defence I must say that at least I wasn’t a bad [at the position/at being goal keeper].

Because you said “I must say AT least”, and the use of ‘at’, it is natural for us to continue this with saying “at x y z” !

Not only was I a hopeless case at playing football, but also at playing any team sports.

Hopeless case is generally an insult at another person, however we usually only say “I was hopeless/I’m hopeless” it is not a common phrase to describe one as, but it is very popular as an insult/description of another

At the school I went to studyied vocational training, I also had the opportunity to do judo.

‘I went to’ is a past act prescribed to location Eg “I went to school”. It is only correct to say “I studied” !

Also as you are implying another opportunity you had (vocational training as the first, judo as the second).

The truth is that I became very interested in everything related to martial arts.

We always say “related” when talking about two things:
“Are you guys related?”
“How are these two things related to each other?”
Examples of relate:
“Can you relate to the text?”
“Who did you relate to?” (Like watching a film and discussing characters = emotional/personal connection)

To make things worse, I was the tallest, which doesn’t mean I was the strongest, at judoka at my gym.

Assuming judoka is a sport ! Would be in this way

This might have been an advantage, having longer limbs, but in my case it was a draw(major) setback.

Stylistic, grammatically we always say ‘setback’. Although drawback is technically correct, it is not popular

Although I was just a yellow belt, and a very bad one, the brown and black belts always picksingled me out to fight (against them.).

‘Singled me out’ = targeted me (subject) (to bully/intimidate/ etc.)

You can say against them but this is implied through ‘to fight’

So formula for one correct use of this phrase: they always singled (subject, in this case ‘me’) out to (verb).

I spent more time on the floor than I spent standing.

Cohesion !

At nineteen years old, I began to working.

Working is correct, however ‘to work’ sounds more natural !

Earning money allowed me to enrol at a gym where I could do what I had always been wishing to since I was a teenager.

When saying ‘allowed’ always clarify the subject after:
“Her mother allowed HER to go out with her friends.”
“My teacher allowed US to leave class early.”

After so many years of not standing out in any sport, finally, I found out that I was really good at lifting weights.

‘After so many months/years/weeks OF (this action/occurrence)’ <- formula

My body responded very fastrapidly and my appearance changed radically; from being a thin teenager, I became a veryto a muscular person.

Or you could say: “from being a thin teenager to a very muscular person”

I don’t usually like to show off.

However I can’t help but saying that I was the envy of the gym.

Wow !

Sadly, all of this stops happeningped long ago.

This would be the only correct way to phrase it!

I had told him that I thought the exercises he was doing weren’t suitable for him and that he should do(try doing/do) another ones.

Either or ^

At the gym, being surrounded by people lifting weights and especially helping my son improve his training made me remember those past days when I used to go to gym.memories.

Not that I was the kind of child who excelsled at sports, quite the opposite.

In my defence, I must say that at least I wasn’t a bad goalkeeper.

Not only was I a hopeless case at playing football, but alsoI was also terrible at playing any team sports.

Usually "not only" is not followed by "but"

At the school I went towhere I studyied vocational training, I had the opportunity to do judo.

The truth is that I became very interested in everything relateing to martial arts.

However, I was as good at doing judo as I was at playing football.

To make things worse, I was the tallest, (which doesn’idn't mean I was the strongest,) judoka at my gym.

ThiHaving longer limbs might have been an advantage, having longer limbs, but in my case it was a drawback.

Flows better

Earning money allowed me to enrol at a gym where I could do what I had always been wishing to since I was a teenager.

After a few months of lifting weights, I started seeing changes in my body, especially in my chest and my upper arms.

After so many years of not standing out in any sport, I finally, I found out that I was really good at lifting weights.

My body responded very fast and my appearance changed radically; from being: I transformed from a thin teenager, I became into a very muscular person.

However I can’t help but saying that I was the envy of the gym.

Sadly, all of this stopsped happening long ago.

AfterConsidering everything I have written, it’s quite easy to imagine what I felt like when a few days ago my older son asked me to go with him to his gym.

I had told him that I thought the exercises he was doing weren’t suitable for him and that he should do another ones.

It was something that I had always wishanted to do since I was in my early teens.

Not that I was the kind of child who excelsled at sports, quite the opposite.

As a child, I remember that I was so bad at playing football, (or soccer as it’s called in some countries,) that I was always chosen to be the goalkeeper.

The way you used commas was fine, but brackets are a bit clearer and easier to read here.

In my defence I must say that at least, I wasn’t a bad goalkeeper.

Not only was I a hopeless case at playingat football, but also at playing any team sports in general.

At the school I went to studfor my vocational trainingeducation, I had the opportunity to do judo.

The truth is that I became very interested in everything related to martial arts.

To make thingmatters worse, I was the tallest, which doesn’t mean I was thehave to mean strongest, judoka at my gym.

Although I was just a yellow belt, and a very bad one at that, the brown and black belts always picked me out to fight against them.

Earning money allowed me to enrol at a gym where I could do what I had always been wishanting to do since I was a teenager.

After a few months of lifting weights, I started seeing changes in my body, especially in my chest and my upper arms.

After so many years of not standing out in any sport, finally, I found out that I was really good at lifting weights.

My body responded very fastquickly and my appearance changed dradstically; from being a thinskinny teenager, I to becameoming a very muscular person.

However I can’t help but saying that I was the envy of the gym.

Sadly, all of this stopsped happening long ago.

I had told him that I thought the exercises he was doing weren’t suitable for him and that he should do anotherdifferent ones.

At the gym, being surrounded by people lifting weights and especially being able to helping my son improve his training made me remember those pastold days when I used to go to the gym.