I say be honest with her... go up to her adn say somethign like "listen, I know you have a boy friend, but somewhere along the line i fell for you... I didn't want to but I did and now i dont know what to do. I mean i dont wanna lose you as a friend but every time I see you with him it just hurts me more and more" just be honest... you may lose her or gain her but atleast u can stoop being frustrated with life