I would make a thread on KMC about it.
Sorry dude, but seriously.
...
Anyway, you want my advice. Here it is:
First you buy rose petals. Thousands of them. You surprise her with candlelight dinner and spend the entire time talking about the things you like about her. As dessert finishes you begin sucking on her fingers. Slowly, and not real wet. Just tenderly. You have all night.
You work this into the most mind-numbing oral she's ever received. Bring scuba gear. You make passionate love to her on satin sheets, and tell her you've never felt more alive.
Then break up with her. Just f--ing lay it on her. Say you know she's been cheating, that you loved her, and now your heart has been broken. Get angry. See if she gives in and admits to it.
If she doesn't, you're probably in the clear. Apologize to her like your life depended on it. Then propose to her. Tell her everything you said tonight was true, and that you want to spend your life with her. This is her second chance to come clean, or at least turn you down, which is all the information you'll need anyway to tell you it's not worth pursuing. So then, you're either engaged to a great girl, or you've figured out that you shouldn't be with her. Win/win, and you can break off the engagement weeks later if needed.
And if any of this works, you most certainly didn't hear it from some random **** on the internet who has no idea what you or your relationship is like.
Post pics.