OP
Just wanted to add to what others have said... you are incredible!
I went through similar with my ex-DH a couple of years ago. Our DTs were only a few months old so I was in the trenches with early motherhood whilst he was entertaining himself online talking to women.
I still will never know how I found the strength to kick him out and stick to my guns, but I'm so glad I did.
Whilst he busied himself meeting new women and living out his fantasies of being a "young and single man" again... I got to work on a divorce and a financial settlement that was very very much in my favour. The fact he was both distracted and slightly guilty worked in my favour because he signed pretty everything over to me. We also had a clean break because whilst he was dating he was running up unimaginably huge debts and didn't have a pension, whereas I have always saved and I have a pension.
I don't feel a bit bad about it, he has never provided for his kids since leaving beyond the bare minimum and has left me raising them as a single Mum, whilst working full... a life considerably harder than I ever thought I'd have.
On the flip side, two years on, divorce is finalised and I now live in my very own home with my two children. Life is hard at times, but I am so much happier now and know that I could never have been happy with him again after what he did. When I lock my front door every evening, it's my sanctuary and nobody who walks through the door is entitled to disturb my peace.
A couple of things to note, (you seem really sensible and switched on so I'm sure you're aware)
His family- please don't be too disheartened if, as time goes on, their loyalty towards him shifts. Initially my ex-DHs family were aghast at his behaviour and swore to support me and DCs in whatever way. I guess the saying "blood is thicker than water" is true, because in time the contact with them waned and now they barely see DC. I haven't spoken to most of them in months/years and given ex-DH barely sees his DC they have no idea how they are doing and don't seem to care. Ultimately they are his family and whilst I know they will always think badly of him for what he did (especially ex-MIL) they did eventually support him.
Dont be surprised if he comes crawling back, it's almost certain that he will, mine has tried a number of times. Then comes the anger when he doesn't get his own way. To this day I don't believe he really recognises the damage and pain his actions caused.
Sending you big love and strength - you've got this!