I was a hole set (center forward) and did pretty well oin my level of competitions.

I’m was supposed to stay at the 2 meter line zone and with my back to the goal.

-I changed it to "was" as your story is in the past tense

-You could leave out the "and" - "I was supposed to stay at the two metre line zone with my back to the goal"

-I spelled "meter" as "metre" - both are correct

I was quite good at dribbling and shooting.

But I mostly played mini water polo which is a subtype.

I'm not sure if playing mini water polo doesn't allow dribbling and shooting. So I'll propose two options:

-If mini water polo doesn't allow dribbling and shooting, then the two sentences should be "I was quite good at dribbling and shooting, but I mostly played mini water polo, which is a subtype"

-If there is no limit on dribbling and shooting in mini water polo , then I propose " I was quite good at dribbling and shooting. I mostly played mini water polo, which is a subtype."

It differs by the smaller field size,pool size with unlimited attacking time.

Field is used for sports like football, on grass

My parents brought me there when I was a child to grow up to have a winner mindset, because my town didn’t have any other strong teams which would compete on national level.

The sentence is very long. You could try breaking it into more sentences.

"My parents brought me there when i was a child. They wanted me to have a winner's mindset, because there were no strong teams in my town that would compete on a national level"

But the bullying, and a toxic relationship with athe coach backed upinfluenced my future dislike for that sport.

-Backed up doesn't sound natural here. Maybe you could use "influenced" or "caused" (if these were the direct reasons for your dislike of the sport)

-You could use "the coach" or "my coach"

I’ve had enough of it.

I would propose "I had had enough of it." due to the story being in the past tense.

I was a hole set (center forward) and did pretty well onat my level of competitions.

This is very minor

It differs by the smaller field size, and unlimited attacking time.