I think that's the second time you have made a point of saying that you trust him '100%'.
I wish you all the luck in the world.
But what a puzzling statement to make.
As others have said, the only thing you actually KNOW about this man at the deepest level of his personality is that he is a liar.
He may be a good man in many ways - you don't really know that yet, you don't have sufficient history with him - but the one thing you do definitely know, from the one 'situation' you have seen him deal with so far - is that he is able to lie, again and again, to the person he loved and married.
Right now, you and he are loved up.
He felt like that about his wife, once. All that amazing stuff... he was like that with her. More than that, they decided it was special enough to get married, and make it permanent. More even than that, he's gone through the experience of having children with her. Nothing tops that, really.
And yet now that he feels differently, having had all that between them doesn't inspire him to even pay her the compliment of not cheating on her and lying to her about the reason all that history has come to an end.
Maybe it's right that their relationship is ended - that's not the point. Maybe she treated him badly too. You'll never know that.
But when you say 'He'd never treat me badly - he wouldn't do x or y' - you are 100% wrong.
He may not, but he absolutely could treat a once much-loved partner extremely badly, and he has. He is.
He cannot be trusted and you would be a fool to trust him. Trusting him is not logical.
I guess you don't want to hear that, as you have trodden the same path. You laughed and loved with your H once the way you do with your OM now, presumably. Now you are lying to him. At some point in the future, you may do the same to your OM.
Anyway. You asked about the highs and lows. I don't know first hand, but I'd say an important part of getting to grips with a relationship that starts this way is to be brave enough to face that unpleasant little truth right in the face. You KNOW he is able to cheat and lie. He could lie to his once-much-loved wife - ergo, he can lie to you. More than that, he's likely to have lied to you already, because another thing you can say about a cheater is that they tend to take the easy way out. He may be right about the way his wife treated him, but sadly it's really very possible that it's another lie to smooth the path, both practically and within his own head.
In the same way, you are now lying to yourself in so many ways about how you have treated your children. Both of you could have finished your respective relationships before starting to see each other, which would have been the right thing to do, and easier and better all round for all the children and your once-loved spouses. But you didn't, because you put yourselves first. But you will lie and lie and lie about the efects of that on your children, to each other and yourselves.
So. It's hard not to see you as foolish when you go on about 'trusting him 100%'. When you parrot the things he's told you as if they are facts. Continue to do that and you may come very unstuck. I hope things work out for you.