That is a personal decision you have to make. It also should really depend on the type of affair. If you cheated on him/her in the first few weeks of the relationship only once and it didnt mean anything to you and you never see or keep contact with the person you cheated with anymore then you can consider not mentioning anything to spare feelings and salvage the relationship but in which case you have to train yourself to forget the incident and move on and never do it again. If on the other hand, you cheated on him/her after dating for about a year and it was an on going sexual affair that lasted for months then coming clean would be a good option as it will probably eat at you for life. Personally if it were me I would come clean regardless of the affair and hope for forgiveness.
