Welcome to the club no one really wanted to be a part of, but since we have found ourselves in the LBS club, it's the best club to end up in, since it was inevitable.
Don't be surprised if your wife wasn't already dating before her big divorce reveal. Many humans in crisis do that. Lie and say they want to alone for a while, meanwhile having someone already waiting in the wings.
You did not cause this and you cannot fix this. She is running from herself and the rest of you are collateral damage. Be there for your daughter, and if you can, for your step-daughter as well. If you haven't had a chance, read the link in my signature "Survival Instructions for Newbies"
Post here as often as you need. We have LBS around the globe and someone is bound to be up.
Take care of you. Put on your oxygen mask. Try and eat something or drink (smoothies, protein shakes) when you can, drink plenty of water, go for walks. You may need some melatonin for a while to help you sleep.