I could never be a hero

After I became a teenager, I realized that I will never be a superhero. -> When I became a teenager, however, I realized I would never be a superhero.

As a result, I decided to be a hero. -> (Good)

Hero, what a powerful, fantastic and charming word. -> Hero: what a powerful, fantastic and charming word.

But since I became a adult, I gradually know that I could never be a hero. -> But since I've become an adult, I know that I could never be a hero.

I am nothing great but a average person leading a boring life and struggling to buy a house. -> I am nothing great, just an average person leading a boring life and struggling to buy a house.

I expect to do what I really want to do but do not have the courage to give up what I already have. -> I have a dream of what I want to do, but not the courage to give up what I already have.

*Nice short essay! Just be careful of your tense agreement. :)

I Could Never be a Hero

When I was a little boy, I always had dreams of becoming a superhero like Batman and thought that I could be one someday. However, after I became a teenager, I realized that I will never be a superhero. As a result, I decided, instead, to just be a hero. Imagine being a “hero”! What a powerful, fantastic and charming word!
Since I became an adult though, I gradually realized that I could never be a hero either.
I am nothing great, just an average person leading a boring life and struggling to buy a house. I expect to do the things that I really want to do, but do not have the courage to give up what I already have.

Hi Alex. Good job! Sadly, such is life. As to your composition, whatever changes I made were merely done to allow your sentences to sound more natural to my native ears. As for being a hero, maybe that hasn’t happened yet, but perhaps, after marrying, you’ll have a child, and will raise that child to be a good and productive member of society. In my eyes, that will allow you to call yourself one, and I am sure that your wife and child will think so, too. Similarly, just living a productive life, one in which you find peace for yourself, yet, contribute to society, can also allow you to be an "everyday" hero. I hope that this helps you. J.