i found out today that she isn't bi, so

Girl, what have you got to lose? Just tell her, make her think twice. If she still rejects you, at least you tried. Not trying is always regrettable.

This must hurt you a lot. I am not gay and am an older guy, but love is love and the inability to express that love to your friend must hurt and probably makes you feel a little bit alone. You just need to convert that love you have for her in a relationship way, into loving her more as a friend which you can express to her without feeling nervous and will only strenthen your friendship. Good luck.