I think I knew that I was a girl in early childhood, based on the scant memories I have from that far back.  That is, I knew I was a girl until I was told that I wasn't.  Mom said I cried when I got my first haircut.  I remember trying to line up with the girls on my first day in kindergarten.  Playing concert flute from fifth grade until the end of high school.  Building a model railroad and historical dioramas (Because I couldn't have dollhouses).  And lots of other little things that, taken by themselves meant very little, but weave them together into a cohesive whole, and -- I'm a girl.  I couldn't wear pink as a child, but when Don Johnson wore pink on Miami Vice, I spent the entire 1980's wearing pastel colored T shirts and polo shirts.
After I was told I wasn't a girl, I tried to fashion an alternate identity I could live in, but that was all that it was -- a male persona that I created and used to interact with the world.  I never thought of myself as a man, because I always felt like I wasn't qualified for the job.  But it was a job I had to learn and I had to do.
I gutted it out living as a man until the anxiety and the panic attacks had all but killed me.  I couldn't stand it any more, so I went in for therapy and started hormones.  I've turned from a sad, cynical, disaffected guy to a girl who smiles every time she sees herself in the mirror.