Wow. Just wanted to pop in and say how sorry I am. My H's affair was with a friend, just a playgroup/mom friend of mine, and that was so devastating. I can't even begin to imagine the double betrayal you are feeling. And to have to work with her? Oh man, you can't escape it, can you?
Try to rest, be selfish with your time (do what you want!), and love on your kids. They'll help you through!!