At what point did you realize that you really didn't want to be a guy?
It isn't so much didn't want to be a guy. I'm not using the "always knew" bullshit. In my case all my friends were girls when I was really young. There were boys around but I always picked girls as friends and I had girl things as a main interest. At some point, you're told there's something wrong with that. So you start doing what how you're "supposed" to act. I still had interest in girl things but knew it was wrong to indulge.
Eventually, I just accepted I was a boy and stopped doing it. But it never goes away and it sticks with you. More than half my friends were girls going into middle school. I didn't know transgender or whatever Was even a thing. I called a hospital to turn me into a girl. I was pissed when they said this was not possible.
Anyways, you get to a point where you research what transgender is, think "that's me". After that depending on how willing I was to do it, I decided to go to therapy to see if my worries were legitimate. It is a long process. It didn't help that people like the people in that gaf thread make you doubt yourself so you don't even bother with it, and end up delaying it (I've delayed mine by five years).
It is less or a moment of "deciding I don't want to be a man" and more "saying fuck it, I'm doing it" after years of internal pressure.