Wow LOTS of Testosterone in this thread!
An Old Guys perspective: Just move on life's too short to dwell on negetive crap you'll only feel bad at the end of it all.
You have to accept some of the blame for it happening as if she were happy she wouldn't have strayed so look into yourself and try and pinpoint what part you had to play and try to not do that again in the next relationship.
In a year or so repeat and again every few years till you're old and can pass this advice along to a young heartbroken guy