Have to agree with that, you are either done or you are not.
Playing a game somewhere in the middle is not good for you (or her, but lets focus on you!).
You can ask yourself, why are you sleeping with a person you don't want to be in a relationship with anymore? Why are you spending time with her?
Even if you decided you just want to be "friends", you will still need some time away (months or years) to let the "romantic" feelings fade.
Sitting on the fence is only going to hurt you and her. Either be in the relationship or be out. Not only does everyone get hurt, it's also really unfair to the other person. While the relationship is a rollercoaster. I think perhaps you don't want to hurt her more than you are?
I think AnvilheadII put it very well once, a break up early on in a relationship can happen once and is forgivable. After that, be done or don't be.
You are sending mixed messages (which has nothing to do with her at all).