Posters should never underestimate the effects of shock. It catches people very differently. Some people are immobilised by it, some feel energised by it and it's very common to feel incredible anger. I expect because you felt utterly powerless OP, you simply felt you had to do something with all that energy and anger. It could have been worse. And it's hardly the main issue, so don't waste any more of your posts defending yourself. If the OW's got any sense, she'll take the blow....
It might be a midlife crisis, or it might be that his fidelity had never been tested before and it was a new exciting experience that he'd have accepted at any time. Sometimes calling it a midlife crisis is a false trail. It's assumed that just because a long-married man in his forties or fifties has an affair, it's connected to a yearning for lost youth, but it isn't always the case. Invariably with men in this category, it's just the fatal combination of opportunity + low risk of being found out.
If you know without doubt you could never get past this, then that's a good thing because it will help you move on and heal much more quickly, but I would advise you that decisions taken while in shock are rarely sound and you should give yourself some breathing space before making anything final - one way or the other.
I'd second oikopolis about why he was still giving the appearance of being committed to your marriage. That's because he was committed to it and didn't want his marriage to end. He's in shock too now, which is why it took him ages to face you and is why he couldn't even talk to your DD on the phone. He never thought he'd get found out and in the cold light of discovery, he cannot believe it's come to this. Right now though his tears are for himself and not you or the kids. Right now the OW is the only one who is in his corner and because he's a weak man, he's reaching out for what is the only source of comfort. If she was pinning her hopes on him, she will be laying the sympathy on with a trowel and unfortunately adding to the picture of you as a vengeful harridan.
Deep down her greatest fear is that your husband will grow a spine and admit that he was using her to prop up his esteem, but never wanted to lose his marriage. If he has got any character, he will be honest with her and apologise for leading her to believe she was more special to him than she is, but he will end the relationship completely regardless of your decisions.
All you can do is tell him it's over and watch what he does. If he ends it with her and fights like mad to get you back, he might be worth listening to and it might even be possible to start a new relationship, a long time down the line. If he takes the line of least resistance and stays with her, then in many ways that's the easiest route for you too. You'll know that he will remain a weak man and not worthy of you.