I'd say you need to send her a card, maybe flowers along with it, saying how much you enjoyed preforming with her, that you felt like the two of you worked really well together...that sort of thing. Then follow that up with looking for other opportunities for the two of you to play together. The "she has a boyfriend" thing is basically a history lesson, he had first chance because he came along earlier than you did...one should never lose out because of a mere temporal disadvantage. :)