Have you tried to look closely at your fear and guilt. Like when does the guilty feeling start... when you are getting ready to go out (i.e. anticipating having sex), during the actual act, or does the guilty feelings follow the sex. Maybe your guilt stems from some unconscious belief that you don't deserve to feel good. Or maybe as Rhap suggested stems from being taught that sexual feeling or pleasure is evil or bad. You might consider when you feel fear the same way. This might help you determine what exactly you are fearful of.

I've started to try and distract my mind when the guilt and fear start creeping in. I don't know if this is a "recommended" technique or not. Just a thought to try. Its help me a little bit.