Maggie - so sorry to read this. I can imagine how much this hurt, on many levels.
I expect you feel that you must have been living in a parallel universe all this time, believing that your relationship was intrinsically fine, but with a few problems that could have been ironed out. Please trust yourself well enough now to affirm those instincts. Please see your H's mindset now as just another stage in "the script" that so many betrayers follow.
They just cannot cope with the truth, so they conveniently re-write history. Pretend that their primary relationship has been unhappy for a long time and reason that if it hadn't, they wouldn't be doing this. This is much more comfortable to live with than the more unpleasant fact that the reason they are having an affair is because of a weakness in them and not their relationship.
They never question why, if they were so unhappy, they didn't bring their grievances to the table. Never challenge themselves that even if they were unhappy, bringing a new love interest into the mix might be the wrong way of resolving problems in an adult, responsible fashion.
They also never wonder why, if they were so unhappy, they didn't confront the situation in an adult way and bring the primary relationship to an end. Instead, they continue to deceive their primary partner and wait to say all this only once they are caught out having an affair.
Your H is even going one stage further than this and is re-writing the future too. Comforting himself that the future marriage would be hell because you'd never be able to forgive and that he might regret a path not taken.
Unfortunately, when someone is in this place, there is nothing you can immediately do to shake them from these delusions. All you can do is assert yourself and make your own choices.
Tonight therefore, I would be telling him that you understand the script he is following exactly - there is nothing new in any of it - but that you refuse to collude with it. That you trust yourself and your memories completely and you merely see his version as an evasion of responsibility.
I would then make sure your actions force him to take some responsibility for once and get him to move out. As we've all said on this and other threads, it's a bit of a cold shock when these history re-writers have to face up to what they've done and go public with their behaviour. You might want to think about some cards you've got up your sleeve - I can't imagine for example, that his head teacher is going to be enamoured with having a love-sick couple teaching a class. Don't collude in their secrecy.
OW will also be following a similar script no doubt. She's probably justifying her own behaviour by convincing herself that he hadn't been happy for ages and that in any case, their love is "so special" they were powerless to do anything about it. The more uncomfortable truth however is that in all probability, she set her sights on a married man with a young child and then did everything in her power to get him away from his wife. Because her conscience won't allow that truth, she has probably been living in her own bubble. Once the affair is actually exposed however - and people start to see through these delusions - it becomes an uncomfortable place for her too.
Remember Maggie that the facts are unassailable. No matter what led up to it, adultery and deceit are wrong - as adults we can rationalise that we understand why it happens, we can even have some sympathy for human fallibility, but none of this ever makes adultery right. Don't ever let them off the hook with that, however much they excuse it to the outside world.
I'm glad you've reflected about his controlling behaviours too. This will really help you if you need to move on and away from him - as it will also help if you ever re-build with him.
I'm sure you've been doing a lot of thinking yourself about what life might be like without him and I hope you can see some positives - it sounds like you can, which gives you a lot of strength.
Keep posting.