I've read all your posts, OP. Firstly, you sound incredible. Secondly, your husband is the kind of man who used to be cast out of communities in the old days for doing what he's done. For a man to leave his wife with their baby and very small kids is the absolute lowest of the low. "Despicable" doesn't even cover it. He will end up haunted with regret, OP, you mark my words. You cannot blemish your life the way he has without it catching up to you. The stain of shame will never leave him. He'll be badly thought of by anyone that knows him, and any future dating prospects worth their salt will run a mile when they figure out the timeline and how old the children were when he left. I'm sure I speak for many women when I say that I could never have warm feelings for a man who'd done this to someone.
He's pathetic, OP. He caved like a cheap table less than four years into fatherhood. Eventually, you will feel sorry for him for being so character-deficient.
As for the OW, words fail me. How can she take a father from a tiny baby and three helpless children? Again, this is a stain on her life and character that will never leave her. She's much younger than him, and she picks a man with a wife and three children? She must be completely mad. Your H is also mad, because companies take a very hard line on people sleeping with direct reports. Your H is not only a despicable cad, he's reckless, impulsive, and as irresponsible as a fifteen-year-old on drugs.
Just wait until she has an "accidental" pregnancy - and she will. Your H will find himself with two families, run ragged by both, and still unhappy.
A much better future awaits you, dear OP. You have done nothing wrong. It's all terribly sad for your children, but that's not your fault and is out of your control. I'm sure it doesn't seem like it now, but it's actually fortunate that this man hasn't wasted any more of your years. You said that you used to have an amazing relationship, so I feel very sorry for you because that is a hard loss.
I think parenthood sends some men round the twist. I have a friend who also used to have a great relationship with her husband, but a few years into fatherhood, he changed out of all recognition. He put them all through complete hell. They're now divorced and she's much happier.
For whatever reason, some men just turn into ice-cold monsters somewhere along the line. There's no use puzzling over it. Best to just work on accepting that this is who they are today - someone you wouldn't touch with a bargepole. It's hard to stop trying to make sense of it, but the truth is that you will probably never understand.
Many hugs to you, OP. Better times ahead.