I dont think anyone is telling you you can stop loving her or wanting to be with you. But she is definitely using you my friend, as you yourself said. I mean you know she has a "boyfriend" yet she is easily engaging in a physical relationship with you at the same time. This speaks to me a bit of her relationship integrity. If she did break up with him for you, would it really mean that much? I dunno, I suppose that is one of the things that I take into serious consideration in a relationship. It would be quite difficult to trust a girl that was with me a long time before breaking off a previous relationship for me (not to mention I've seen this happen with bad results many a time before with friends).

Good luck my friend