You made a mistake. Sadly you have to face up to that. I know this goes agaist other's advice, but maybe you should tell him. You're engaged and want to spend the rest of your life with this guy, if you just try to pretend nothing ever happened, it will eat away at you, and most relationships built on lies won't hold for lon. The truth always finds a way of coming out. Ultimetly the decsion is yours, but if you're going to learn from this you have to face the consequences of your actions however bad they may be. Plus you never know, you may be able to out this behind you both.