I know it's hard to believe, but you'll look back on this one day and be glad it happened. Had the same situ with my 1st H. Met young, we were each others firsts etc. 16 years in, I found out he had been unfaithful with several OW. Took me 4 years to leave!
Within 2 months, I had met my now DH, and we are so happy. Been together 17 years now, and he's so much nicer than my 1st H.
Be very wary - once he knows you are leaving, he will turn nasty - mine assaulted me twice. Also, be prepared for him to try to take the kids, and try to keep as much of the money as he can. It's every man for himself.
When all this was happening, my best friend dumped me. Was really weird. I managed to look at his phone, and he was shagging her too! I called my best friend and told her that I knew they were having sex, and that one of her other friends had tipped me off. She was left wondering who had betrayed her, and lost a few friendships in the process - sorry, not sorry.
When you do tell him that you know about the OW, I would tell him that the OW told you herself, and that she said she would deny this if he asked her about it. That way he will never know if the OW was actually the person that blew up his marriage.
Fwiw, my 1st H went on to cheat on his next partner too. And I think he's cheated on his current partner as well. Grim.
Another warning - his family are likely to stick by him. My MIL and SIL's of 20 years never spoke to me again, because in their eyes he had "just been having a bit of fun, whereas Blondie was breaking up a family". Twat's the lot of them.