Hell, why should she leave? She's got you wrapped around her finger it seems. According to your own very words she has pulled this before. This does not seem like a healthy relationship. It also seems that her respect for you is in question if she jerks you around like that.
Respect is darned important in a relationship, it's what mends breaks and keeps actions in line. From what you've said, it doesn't seem like it's that strong. Plus, you've admitted to "begging her to take me back". That isn't right; if you pursue this relationship further she'll be the one wearing the pants, and you'll be stuck playing games according to her mood. I don't envy that, to say the least.
The general feeling seems to be that you shouldn't pursue this further, for different reasons. I think you need to keep your own worth in mind, and respect yourself and ditch that woman for someone who may hold you in some regard. Or no one, it's better than being jerked around like that.
That's some .02$ you didn't ask for, but whatever.