[18M] How do i know for sure I'm trans?

I'm 18 and a male and I've had strong urges to want to be a female for months even going on a year now. Looking back, even as a child there were times were i pretended to be a girl and I enjoyed it. I love the idea of being a woman nd lately I've just felt so out of place as a man and I don't enjoy it.

How do you know that it's the right choice? Like what if I make a mistake? What if my friends, family, but most importantly my girlfriend, don't accept me anymore?