You're going to grieve for a while, accept that but you can also start GALing. Do things you've always wanted to do that you didn't have time for, dance class, pottery, acting, whatever makes you feel like you.
Let H do his thing. Nothing has really changed, except you now know you're not going immediately to the poorhouse. He'll continue to see OW as long as he does, just as Georgiab says but as far as it being serious, probably not. He may think so but we all get that glimmer when in a new R. If you think you want to remain married to him, grab some popcorn and let it play out.
Take care of yourself. What are 3 things you're doing for yourself? What are 3 GALs you are going to start next week?
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss