You dumb ass. Now you've ****ed up your chances with the other girl, whom you don't know to be a total bitch.
It's too late to undo it now, but you can still salvage the situation. Hit it and quit it, dude. Tell her she's hot, and the sex was good, but she's not worthy enough of respect to have a lasting relationship with. That's probably the truth.
She's just going to use you, again, so you might as well use her, first. First you should be honest with the other, nicer girl, and try to explain yourself. If you ever get in a relationship with her, this would be bad dirt to have under your rug. If she can't get over this, you'll have to accept that. If that's the case, then you may as well continue having sex with your ex, but only if you can do so without getting emotionally attached. No matter what she says, do not accept being in a monogamous relationship with her; she should be treated as a **** buddy. She seems to be an attention-craving egomaniac, so I'm guessing your new attitude will cause her to try harder and harder to get you back. Not because she loves you, but because you hurt her ego. The more you don't love her, the more she will pretend to love you. If she is capable of learning her lesson, this will teach it to her. I don't think you can ever know if her love is genuine, though, so I would not risk taking her back, even if you think she did learn.