You seem to have put yourself in quite a tricky situation! Having casual sex is never a ‘no strings attached’ situation especially for us girls. Just like love and hate there is a very fine line being sex and feelings; one does not come without the other. It is almost impossible for girls to have sex and not have feelings somewhere along the line.
It seems that either way you look at it, you cannot win. I would advise you to think about what you want and really think. Know that whatever the decision is someone might get hurt, you included. So please take every party into consideration. If you were in the position of the girlfriend would you want this to happen to you?
Remember that there are so many other guys out there that are better and truly deserve you. And I am sure it won’t be long before you find him.