I've actually been through something similiar. We weren't engaged, but it had crossed my mind. Either way, I'd never so much as looked at another woman and ye olde girlfriend gets smashed and sleeps with some guy she used to know her hometown.
Now, I loved her and was determined that I was going to forgive her and we were going to move on with our relationship. However...I couldn't get it off the brain. Every time I looked at her smiling, laughing, talking, all I could think about was her naked in bed with some other guy. It interferred with my schoolwork and made my life sheer hell. Eventually it dawned on me that if I was going to hurt this bad that I might as well be spending that time healing other than reopening the wound--which is what I was doing every time I talked to her and spent time with her. I told her I needed some space. Since she was the one who'd committed the indiscretion, she really couldn't deny my request.
So during my "break" I didn't call her, didn't talk to her, didn't see her, NOTHING. I thought about her a lot, but knowing that theoretically I *could* contact her if I wanted to helped a lot. That was my detox. By the end of two months I felt better--but I knew that if I tried to patch things up with her or contact her in any way I'd be right back to square one.
I didn't want to go back to square one.
And inside, of course, was that seeping knowledge that while I'd been purifying myself of her poison she was out there with her friends partying and doing who knows what in my absence. Later on I met the girl I'm with now and realize that I am much better off. Plus, had I stayed with that girl we'd probably still be trying to make it work and I wouldn't have the healthy and very fulfilling relationship that I have now.
It's up to you how you want to work this, but I've played that game and couldn't get past the cheating. Just be glad she did this BEFORE you guys were married. She saved you an expensive divorce.
There are very few personal problems that cannot be solved with a suitable application of high explosives.