Who can decide whether I'm a girl or not ?

I have ocd so it's kinda hard. I do bae things. I try to be a girls' girl and I try to not hurt anyone, but I make mistakes.

Yesterday i had an argument with a stranger online who has been supportive before but then he said I always make excuses to not transition (which is my ocd making "what if...?"s) and I'm too evil and he said im just pretending to be a girl.

So, I can ask my friends. I can ask Google. I can ask you. But who can ultimately say if I'm a girl or not ?