and he's been crying all weekend over how much he's hurt me Well it didn't seem to bother him too much for the best part of two years while he was doing it did it?
I don't think he is crying over how much he has hurt you as much as he is crying over how much he has now been found out, how deep in the shit he is and how much he could lose his nice family life. Nothing to do with hurting you.
the OW seems to mucking everyone about with her lies. So suddenly it is all the OW's fault? Typical when a cheater is caught out that suddenly is is all the other person's fault.
What about the 2 years of lies he has told you. Sleeping besides you, Christmases, birthdays? All the times he has made arrangements to be with her and sloped off for a shag when he could have spent time with you as a family! Funny that he is now purer that pure and she is the liar 
Is he taking any responsibility for what he has done?
You are 100% right. He would still be doing it if they hadn't been careless and got caught.
You cannot fix him. You cannot make him look into himself and see why he allowed himself to do this and what he can do to change himself to stop this happening again. Only he can do that.
Take all the time you need and decide what is best for you and any DC. He does not get a say now. Whatever decision you make it up to you it is not 'wrong'.
Go and see a solicitor so you know exactly where you stand legally if you do decide to separate. If you don't need to use the knowledge that is good, however knowledge is power and a little protection.
Be prepared for the swinging back and forth for a long time.
Please get yourself to a clinic and get a full check for STIs. Ensure he does too if you are sleeping with him 
Good luck /hugs