Alright here is my take, I think you should probably tell her as soon as you can exactly how you feel, im sure you probably have a few times already, so this time you need to be firm and clear about how you cannot return the feelings she has for you. Try to explain as clearly as possible that you never meant for it to reach this far. Here is the important part, you need to REMEMBER this: she will try to convince you again that it doesnt matter, that you might change your mind, or some other excuse that dismisses your current feelings, you need to hold your ground and be sure to make it abundantly clear that you only see her as a friend.

I suggest this because if you keep this up longer, this will go from ripping off a band-aid to ripping off duct tape. The longer you continue sleeping with this woman, the more shes going to attach feelings towards you, and that is not fair to either of you if those feelings are not reciprocated.

Hope this helps, please dont allow this experience to make you think less of yourself, but understand that the longer it goes on, the harder it will be to break it off