I am 21 and when I was 13 I had a very huge crush for many months on a boy. I thought back then I have to tell him I like him, because if I won't I will regret my whole life that I was such a coward.

Spoiler alert: I wouldn't. He didn't feel the same towards me. It hurt for some time but I got over it a lot faster that I thought. But it gave me important experience and courage that I'm still proud of. The boy also was polite, didn't laugh at me or anything, which I'm grateful for. Took my feelings maturely.

All problems you have at 13-15 don't matter once you are 17-20+. Same can be said about fail and success. So no worries OP, go ahead and ask her out. Even if it won't work out, you will get experience and important skills. If it does you will be very happy. Sounds like win win.