Then it sounds like you know what you need to do. It just really sucks sometimes.

One thing I kept telling myself that helped was that the person I knew didn't exist anymore, and was replaced by this new person that I wouldn't want to be around anyway.
From what you've said, it doesn't sound like this new person is anything like what you'd want to be anyway. Also, keep in mind:
You can't be more "new" than the AP
You can't be more forbidden than the AP
When he's addicted to the affair, that takes all priority, just like a drug addict. Even if you were "more like her" (ick), you can't be as new or as forbidden.
Just be the best you that you can be. For yourself.