Reading your post felt like me going back in March, April, ... until September this year. When I first found things out I thought he was doing this from March on to be surprised it was from November last year on. He was in several date sites and cyber porn sites. We had several conversations just for me to find out kept doing it. He stopped the online thing after I found about the cyber sex as things got ugly just like it is for you now but then went straight to a PA.
First thing I can tell you. Stop monitoring him immediately. If he has done it yesterday it's most likely he will do it in another way or different emails. Don't cancel yourself his accounts. Let him do it when he feels like doing it. Don't take the full responsibility to end his cheating. It's his responsibility.
Right now he's upset to see you suffer but I will be straight and dry. He is not sorry for what he's done. He is testing you to see how much you can take.
This is not the point where you need to reassure him. You need to reassure YOUR feelings to him. Made that mistake of reassuring him while he was doing these things and I assume it only made things worse because he put himself in a victim's role like: Poor me! What you do about his validation issues and his cheating are two different actions and moments.
Now it's the time for you to read DB and put it to good use as soon as possible. I am sure if I had done it before my H PA and not after, things might have been different.
What you need to do now it's very, very hard. But if you want your H back there's no other road.
