I'm having difficulties deciding which one of you is the most hopeless masochist: the girl who doesn't quit failed relationships, or you the guy who doesn't even take the most basic steps in order to get what he wants,... In a sense you are a good fit together.

I think the best would be to cancel the whole thing, or not go yourself, and then arrange a meeting with her where you'll ask her to leave the other guy and go out with you. But of course, being the person you are, this won't happen, and you've just earned yourself a few more weeks/months of melodrama.