I ditto everyone else's posts about your situation. Instead of fighting for his family, he ran, he stepped outwards to another woman. He wants what he wants, the h*ll with his family. He is very selfish. When WAS's leave the marriage, then it's all on them, 100%, as far as I'm concerned. Marriages aren't perfect, it's hard work, it's a lot of communicating (talking and listening) and give and take. The LBS is not a mindreader, to just know through some ESP that someone is unhappy, especially if they're not acting that way. For instance, my H is very passive-aggressive ... he says nothing, and then just does what he wants behind my back.
I agree, there was nothing you, on your own, could've done to keep your family whole. It takes two committed adults to make a marriage work. And he is making a lot of excuses as to why it didn't. You can assume whatever is coming out of his mouth is either untrue, or a manipulation of the truth to make him look like the victim. And it's so easy to re-write history. I'm sure OW only knows that skewed history.
Sometimes, we LBSers (I include myself) have to just realize that the men we thought our H's were, was just a figment of our imaginations. They play at being adults, good guys, caring fathers, but when the chips hit the fan (and OW comes into the picture), they have all kinds of excuses dug up from some small thing you may or may not have done, in some distant past, why they are leaving you and by extension their children. They forget those vows very quickly. OW will find that she will also go through this, unless your H doesn't get help. I think, IMHO, your dad is right ... don't give on anything, because to your H, that is a crack, a weakness he can exploit. And, he will pound on that weakness until you give on another thing, then another, and so on. He must face the consequences of the choice he's made. Let your lawyer/dad do his job, and you stay out of it. Don't have anymore meetings. Your H isn't interested in smoothing things out unless it's to his benefit.
Be the best mom you can be. Be the woman (and I suspect you already are) that he will kick himself for leaving. I know it's hard when you love someone, but see this as tough love for him. He won't learn anything unless you are tough. BTW, I'm only coming to these conclusions after 7 years of being Miss Nice to my H. I've wasted a lot of time, while he was playing me like I was a puppet in his hands, spinning me around until I didn't know which way was up or down. I've cut the string now. I wish I had a dad like you have. Mine passed away a long time ago. Use that resource to the fullest ... I'm sure your dad would love to know that he's protecting his daughter, and she's allowing him the honor of doing that.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim