I'm not pretending to have this sorted, it's a demon I wrestle with a lot, but in my "saner" moments I think this:
The OW may be prettier, skinnier, more accomplished BUT she does not have a good soul, she does not have a kind heart, she does not have a gentle spirit. And in the great big scheme of things THOSE are the things that count.
To do the things she did - to sit and listen to him phone me, knowing she was about to go and have sex with him, to sleep in her bed with him 40 times, knowing he had a family at home - those things show me she was NOT a nice person. And in my books, niceness counts way more than all the other surface stuff.