It’s good that you’re finding your anger. Obviously not enjoying being at his mums that much! Be prepared for more back tracking as he realises parenting isn’t that easy.
a couple of things you might want to put in your back pocket:
1 - I’d let the MIL know he’s been spending a lot of time time with colleague X ( make sure she knows her first name). Don’t accuse him of having an affair, but maybe mention that he’s gone out of his way to drop her off, rather than come home to spend time with his kids.
it plants a seed. So when the ‘new’ girlfriend is introduced, mil will know she was on the scene
during this break up.
2- think about what you want. Is it kids most of the time? Or 50/50? Courts favour 50/50, but so do dads who want to pay as little child support as possible. Just remember, his love life will take a hit when he has to step up and parent, so I’d be going for 50/50. No bigger passion killer than other people’s kids- I don’t know many young single women who’d relish spending their weekends with three young kids, so go for 50/50.
3- this might feel quite painful now, but trust me when I say that if he does run off with this other woman, he will be going out of the frying pan and into the fire. He hasn’t left you because you’re not ‘enough’- he has left because he’s realised that life with young kids is exhausting and relentless. He is craving what life was like before you both had kids.
he thinks he has it with OW, but the jokes on him. If she is younger with no kids, guess what? She will want the kids. Fast forward 5 years and he’ll be in exactly the same situation he’s in now! You, on the other hand, will have older primary kids, life will have calmed down, you’ll have your career, and a social life because he will have the kids half the time. He will probably have a couple of kids with the OW, he’ll be exhausted, skint and the OW will be nagging him to pull his weight.
Karma can be a beautiful thing!