That's pretty bad... Then again any cheating is bad.

Once that trust between the two are broken, it'll never be the same... You might be able to look past it, but I'm sure if you look at him, you'll see him in a very different light... I don't really what I would do to be honest... 12 years is quite a bit of time... It also depends on the person too, and more importantly as to WHY they did...
People that get all their needs fulfilled and are happy with themselves don't go off looking for something else...
What is it you want? Do you think you can look over this over time and fight the animosity that comes over you whenever he does something unpleasant (at least to you)? It's a long uphill battle, but some believe it's worth it because the other person gives them reason to...

Enough of my babble... I need more cofee