Everything Snax said is right.
The only thing i'll keep pressing is not to go after the guy she cheated with. This should never be done in ANY situation in which there is someone cheating on someone else. Go punch a pillow, you'll get the exact same result - only without the potential of being charged with assault and being sent to the slammer.
And it's tough, but just as everyone said, you have to leave this woman or you'll become neurotic for the rest of your life for as long as you're with her. Best thing to do is sit down with her for a talk, keep a nice collective and calm tone (DO NOT under ANY circumstances explode or yell or even give the slightest hint that you're angry), and tell her it is over and explain why. Then get the divorce process going.
One final tip: think logically, not emotionally. That's one of the things that separates males males apart from females. Never do anything based on emotion, always use your brain, always use logic.