1. She knows she can take advantage of you, so she does.
2. Two can play that game. Since you're going through a dry spell, why not use her too? Plus, she's the sloot with a boyfriend, and you're single, so she's doing wrong, not you, really.
3. Once you have found someone else, totally kick her to the curb. She's not worth your time or worth risking a real relationship for.
4. Obviously, she can't be trusted if she's doing this to her current boyfriend. (This is where I am a hypocrite, but this isn't about me. lol)
5. Stop paying for her!! Seriously.
My above advice might seem like the "dick move" approach, but honestly, for a person like this that you're describing, this is what you need to do. If she was a good, decent girl, I'd say go a different route probably, but clearly this isn't the case with her. She's just one of those chicks that wants her cake and to eat it too and you're paying for the cake on top of it. Don't invest anything into her. She's getting what she wants by you agreeing to "hang out" with her and you're getting the same in return. That's where it stays fair, but once you go paying for shit then that makes things worse and you're putting in more than you're getting in return.
But, I definitely advise dropping her like it's hot once someone worth it comes along.