It's not your fault;it's about him.
It's amazing on here how many men get married and have kids but don't want to step up to the responsibility of being an adult and all it entails with it.
All you and many of us end up doing is "adopting" a man child who's looking for a replacement mother.
I find these men have often moved in with one woman to another whether that be their mother to their girlfriend or girlfriend/wife to another woman.The common denominator is that these men expect the women they live with to do everything whilst they dip in and out as they wish.
He didn't fix things or do anything because he's irresponsible and immature.
I'm betting he moved with you from your mother in laws house and has probably moved in with the other woman.
I'm sure it's all fun and exciting and lots of sex until reality sets in and the other woman realises she's now stuck with a lazy twat who doesn't pull his weight and isn't capable of "adulting" and you can bet he expects her to do all of the "woman's work" when he has the kids too.
Believe me the grass isn't greener on the other side and usually smells of bullshit.Don't be jealous of the OW of hateful because she's done you an absolute enormous favour by taking him off your hands.He's sloppy defective seconds.
You absolutely do not want him back.I understand that right now you're absolutely devastated and grieving for the loss of your marriage and things are very difficult right now but you can and will get by eventually.You need time to process this and heal.