Firstly, I’m so sorry you’re going through this. That sense of the world shifting on its access is ghastly.
Secondly. You WERE happy, both of you, but on different terms. He was happy because he was having his cake and eating it, that’s the difference. Don’t at any point blame yourself for not realising etc. There’s no shame in trusting things are as they seem, but there’s huge shame on him for taking advantage of it (and remember he’s done it not just to you but the OW as well, over a long period of time, so it’s not a one-off behaviour).
Thirdly, as someone else has said, at some point he will turn nasty. Sorry. It might be telling you you’re being selfish or cutting your nose off to spite your face; really what he means is that you are now the reason (in his head!) why he is unhappy and having to deal with all sorts of fall out. Don’t fall for this.
Lastly, best of luck - like many others I have been thinking of you and trying to send good wishes your way since I first read this. You’re doing brilliantly. Big hug.