If she is dating someone there is prob no chance of it working out. I was in such a similar situation, but what made things worse was the chick who was dating some guy also had feelings for me but couldn't get herself break up with her bf to be with me. Literally was miserable for like 6 months, fucked up triangle, but finally found a new chick and am happy again.
Just one piece of advice, tell her and be very straight forward. But DO NOT try and still be friends with her, because she prob will want to but you will not get over her if you're still friends. That was my biggest regret this year, and I don't want you to do the same thing. Hopefully you will have better luck than me and she will actually be with you, best of luck brah!