Don't give up Legs, you are winning, although you think you arn't.
What's really happening?
It's been 4 months since he's been properly holed up with his younger model, the massive ammount of years he's been with you have accumalated because you fitted together , however you muddled along it was because you fitted together, it may have seemed easy or univentful but it was easy for both of you. Long marriages like this being friends just evolve and many times the love is taken for granted, especially by the men.
His conversation with her must be boring, I'm sorry but the general lack of life knowledge with someone half your age gets boring and annoying quite quickly.
He is communicating with you now not to be friends and hoping you are now at the point where he thinks you are getting 'over it', no he wants now to see your fight for him, he wants to see you are distressed and missing him, pinning for him. He may well be wrapping that up with talk of knowing how the kids are and their birthdays but seriously legs do you believe that, I bet he wasn't sentimental with them at all when you lived together.
He's bored for 'your' type of talk, he's getting bored of his pretend willy and the realisation is just setting in, he must get you to be horrible to him now, because then everything he has done will have made sense, (he was right and you were a horrible person).
Believe me you are winning, the exact same thing happened with me, he would come round on the pretense of something else have a huge argument with me and then go away idolising the ow. When I went total NC he started to question everthing, this is what ended his relationship, the fact it was now just singular and I wasn't in the mix.
You are down, his messages were designed to do exactly that to you, he is reminding you of him, please don't think you are losing at this point, no he is just realising what he has lost, I bet he doesn't even realise that himself.
You are not losing, you are at the beggining of winning.
Please don't get down, it's him whose becoming 'down' and needs reassuring he's made the right choice, don't give him that reassurance.
It's finally time for him to grow up.