I'm willing to bet all those giving it "he's allowed to move on too" have NEVER been in op's position, and more they're either still with their first ltr and think this could never happen to them and/or they are the 2nd wives/ltr of their partners
The person that's behaved the worst as a parent here is the father who betrayed his kids security for a leg over!
Op I've been there, it's a kick to the gut indeed but you will gradually find it easier to deal with.
The difficult part is supporting the children in dealing with it because it's entirely normal for it to throw them too! Especially if it does mean your ex becomes even less involved as a father which unfortunately is very common.
My ex now has 5 with 2nd wife who was also OW and also fell pregnant with 1st just before we split, so in the immediate aftermath of us splitting after me discovering the affair I had to deal with all this and more.
With each new baby ex made less and less effort with dd, to the point that upon receiving the news about numbers 4 and 5 her immediate reaction was to throw up and wonder if she'd ever hear from him again! (Which she barely does)
I've also had the additional pain that I wanted to have a big family but medically couldn't after dd, he was never that fussed about being a dad and has been a shit one to dd.
It's shit! And frankly comments like "he's allowed to move on too" are in this situation incredibly arrogant and unhelpful.
"I think it's selfish and irresponsible to have more kids if you are a shit parent to the ones you already have. Kids need a lot of time to adapt to the new 'normal'. That should be sorted before anyone goes on to have more babies." I totally agree!
I don't agree with bad mouthing the other parent excessively which you admit you did, nor do I agree with being falsely positive about them or any developments that can and will affect them, you need to adopt as neutral and supportive (of the children) a stance as possible.
I've never blamed ex's DC for anything, they're innocent children who've done nothing wrong. I've even spent days out with and babysat them on occasion, not for ex's or their mothers benefit but for dd when she wanted to spend time with her siblings.
Unfortunately ex's actions since mean dd now hasn't seen her siblings for years and she misses them greatly.
Fwiw I have it on good authority that ex & 2nd wife are both utterly miserable, he's continued to cheat on her repeatedly, she's reluctant to leave, apparently one of her concerns is how he'd be as a dad given how he's been with dd, but she doesn't trust him, at one point he wasn't even "allowed" a basic text and call only phone, SM accounts or private email address! At this point I understand she's given up trying to stop him cheating and just puts up with it - though occasionally throws him out temporarily.
I had my revenge fantasies as I suspect many cheated on spouses do, I'm now very glad I didn't act on them, because now all the misery they have, they only have themselves to blame for it.
And yes ignore his rewriting of history, my ex tried that and even his own mother called him on that bullshit!
It's shit, it's hard but it will get easier 