There are a few basic things that I would do. First of all, cheating on her is a bad idea. Eventually you're going to have to give up one or the other, so you might as well do it now. It will only get harder to do. Before you do anything, you need to figure out if the other girl you like is someone who you really want to be with, and not just a physical thing like you said before.
If you don't like the girl who you're with anymore, then you shouldn't be with her. It's that simple. Even though you just got back together with her a week ago, it still would be better to break up with her now, instead of when she thinks that you like her again and that you want to be with her. She was already wondering why you got back together with her again, which means that she'll get suspicious that something strange is happening.
No matter how much you think that you won't get caught cheating, you will. I don't know how, but something will happen like one will call you when you're with the other. That would cause you to lose both of them instead of just one.
Since it sounds like you don't want to be with your current girlfriend, I'd reccommend just dumping her and being with the other one. I hope this helps. Feel free to PM me if you need any more help. Good luck!
edit: This may seem a little rude, but can someone
please help me?