First off, You should find out if she really cares for the person she is dating. If she does she might not want to know if someone else likes her, that being you. It could complicate things for her and your friendship. Play it out a little longer.
Reading throughout the thread you have made it seem like you can't hide how you feel anymore. Sit down and talk to her, face to face. No feelings get mixed up that way. Tell her you want to remain friends no matter her reaction and you absolutely understand if she can't return the feelings.. then tell her. Maybe you'll get lucky and she has a secret crush on you as well.

Good luck.