I have been trying to work on the samething with one of my friends, except I screwed things up about 6 months ago and we are talking again, we started getting close after she broke up with ehr boyfriend. I have feelings for her, when I told her, she said that she doesn't feel the same way about me. I am still devistated and now I am still trying to pick myself up, that was a week and a half ago. I have been told to jsut be there for her as a friend, if she doesn't feel the same way about you, try to move on then, that is the hardest part. See what is hard in your situation is she is already with someone. I am not sure what I would do. try to talk to her about your feelings, but if nothing happens then back up, you don't want to mess up a friendship. That is what I am trying to do, but my feelings are getting the most of me.
Good luck!!!