Sounds like oneitis to me.
Prescription:
(1) realize she's using you for an emotional crutch, and that she doesn't want you, just wants the unconditional support that she should be getting from her boyfriend.
(2) realize that the only time she's possibly going to turn to you instead of her current fuckbait is when she gets hurt, and you're going to feel like a complete ass if you take advantage of her then.
(3) go get some action somewhere else.
Understanding "ladder theory" can help, too.