Been in your situation, bro... My ex did that, with a dang Calvin Klein underwear model who was in better shape than I was, and was 10 years younger. She confessed the same night, and she was all upset, and I forgave her for it. She didn't do it again, and it wasn't the reason she's my ex. It stabs all the way to the bone; I know. If you don't think she's your future, this is a great chance to end it with you on top. If you do want to save it, find out the real reason why she did it, and work on that. Blaming yourself or her, or punishing, won't fix it for sure. Cheating in a good relationship happens because something needs to be worked on, and isn't being. Cheating can also be a sign of a relationship that shouldn't be.
Charles