Oh man, you're going to get a lot of tough love for this one. Don't worry, you've come to the right place. You remind me of myself 2 years ago. Where to start...
1) Never tell a girl that you have feelings for her, at least at not before you've slept with her and you are in a committed relationship. The "confessing your feelings method" of seducing a girl only works in Hollywood movies, I'm afraid. In real life, you have to take ACTIONS and see if she complies. For example, you like a girl? Invite her out for a drink alone, just the two of you. Try to kiss her and physically escalate from there.
2) The girl you talk about in your thread is playing both you and her bf perfectly at the moment. She is, as we say on here quite a lot, "having her cake and eating it." That is to say, she is enjoying the attention from both you and her bf simultaneously, whilst both of you are pining over her. I'm not sure how old you are, but don't believe her when she says she hasn't done anything physical with her bf. The chances are, if they're together, she made out with him as an absolute bare minimum, and very likely slept with him.
3) Most importantly, this girl is neither going to leave her bf for you (as you yourself say in your post) nor complying with your physical escalation beyond making out. Therefore, whilst the girl is getting emotional and physical attention from 2 guys (see point 2), you are getting precisely NOTHING out of this. Just think about that for a moment. Nothing whatsoever. Don't you feel you deserve to have a hot gf all to yourself to sleep with? Why settle for less than you deserve?
On the basis of the above, your best course of action is to keep contact with this girl to a minimum and see other girls. Scroll down to the bottom left corner of the page to a link called "DJ Bible" and start reading- it'll make the advice you receive make much more sense.
Good luck!