Hi Lily, I'm almost 8 yrs down the line from where you are. I too was with exH 25yrs, married 23 when he met OW.
Today I'm happy, in retrospect very happy not to be married to him anymore - though I have some real, and very happy, memories of our marriage. My advice would be to detach as much as you can. You have no control over his emotions and choices, he will be justifying it all to himself anyway. I made the decision (within a couple of months) that if he wanted the marriage to be over HE had to do the leg work. He divorced ME for unreasonable behaviour, I didn't even have to read the petition, just say I'd got it. (I read it years later).
We were divorced within 6 months, he married OW 5 months later. All over in less than 12 months.
It meant I could get on with my life.
ExH and OW have a very different marriage to the one he and I had, but when I met her last year for the first time she asked if he was very moody when he was with me. 

lol, the leopard has not changed his spots and their honeymoon is over.
Detach and ditch him would be my advice. Learn to live again and love your new life. I'm planning a big new adventure / move for 3-4 yrs time, one I could never have made if I was still with him. Life is good. Oh and the unreasonable behaviour thing caused incredulation among family and friends, they all knew the truth, he is the one who crashed in their estimation, not me. I remained reasonable, pleasant, and maintained all of my good relationships with his family - who after 25yrs were my family too. I still see more of his family than he does, get included in events etc. My choice. Good luck, remember, detach and ditch as soon as you can, it's healthier for YOU.