Wow, what a tough spot to be in. I can't offer any advice on how to deal with your D but I can share my thoughts on dealing with your WH.
You need to come down and come down hard on him. You not only need to go dark but actually pitch black. You need to move straight the the last resort technique in the book. You need to completely cut him off from your life (except for taking care of your D) so long as he is going to treat you this way. He needs to choose. Its either you or the OW. He can't have both. Right now, you are allowing him to come and go as he pleases and that's not cool. You deserve better than this. You need to 180, GAL and detach. You need to emotional divorce him. You need to see that your M has died and start finding out how to live your life without him. I know this sounds harsh and counter intuitive but THIS is exactly how you will have a chance at saving your M. He needs to show that he is willing to completely end his R with OW and go 100% NC. Only then can you consider working on your M.
Me: 48 y/o W: 47 y/o Together: > 20 yrs BD: Dec '15, then S 2nd BD: Mar '16, then I filed for D April '16: started piecing