what do you want with her?
she doesn't trust you. you have a GF, but want to fuck her also. and she's attracted to you, but has no reason now to think that it won't end up in some drama.
be cool and straight with her. If she asks about the GF, be straight. "yeah, I do, and that's it's own thing. it's not great, but i'm doing what I can. but right now, i want you, and only you."
women understand cheating as long as you can handle it like a man. and no matter what, never get frustrated if a girl isn't in a hurry to fuck you. remember, if she's in a hurry to fuck you, she's been in a hurry to fuck the guy(s) who were there before you, and god knows where they've been, y'know?
