Sometimes I feel like I don't know who I am.
What do you mean you don't know who you are?
The problem is that I don't know who I am.
You are who you are, it is good to be yourself. Trying to be someone you aren't will only leave you confused.
It's just so hard to be myself. I don't understand who i am and how long does it take to know me?
It can take you a while, but once you actually find yourself then you'll feel much happier. You shouldn't give up on that search.