This might give you some hope: I am not a parent (feel free to tell me to leave if it's not my place to comment here), I was actually on the other side of this. From when I was around 14-18 I fell into this too, thinking I was "really" a boy because I didn't fit in, wasn't feminine, was uncomfortable in my body etc, so it must mean I wasn't actually a girl. I'm 20 now, I came very close to going on hormones and getting surgery, it's scary when I think how close I got. My parents told me to wait until I was 18 to do anything medical, I reluctently agreed, and by the time I was 18 I started to have doubts. Very small doubts, but that's enough, because if you keep thinking about it you run into contradictions immediately. It takes time, but maybe (hopefully) your daughter will find her way out of it too.