I too used to deal with guilt and couldnt let things go. You're human and allowed to make mistakes. You slept with a man, had no feelings for him and now feel guilt. Were you using this man to fill a void in yourself that you have not yet identified?

Do you believe that you deserve happiness? You should applaud yourself for being honest with your fiance. He forgave you, now forgive yourself. I hope you find some peace, and if you cant seek a qualified therapist. I say this because I think this need to punish yourself will haunt you until you deal with it.