You have crossed the lines of good behavior because you have spent far to much time and attention on her, and now you have a mess. She doesn't know what to do, so tell her this isn't right or healthy, and leave her alone and you both do other things, but NOT with each other.
Yeah it sucks and will hurt for a while, but at least you have a chance to think of the right thing that should be done, without each others influence. That has to be better than keep going down the unhealthy confusing path you are on now.
I mean even if she dumps her boyfriend, you already know what she is capable of doing behind a boyfriends back don't you? How old are you both, and where the heck is this boyfriend that she can spend so much time with YOU in the first place?