Don't tell him. As babygirl said, either you are not ready for marriage and maybe did this as way out or you were just acting on impulse. Either way, telling your fiance' will just hurt him and satisfy your need to disclose to reduce your guilt. Tell a therapist so you can talk about it and figure out if this was just an error in judgment or if you really don't want to get married. However, telling him will be satisfying your own needs and will hurt him.