I found myself in a very similar position a few years ago, but with a young baby. It's really shocking, and absolutely devastating. I really feel for you :( and [anger]. You sound like an incredibly strong woman, with a great support network round you. It doesn't feel like it now, but you will be ok, the pain will go, and you'll be so much better off without him.
((((Lou))))
I took mine back about 3 times, and he always played me along (using the same script as yours did - 'you'd never forgive me', telling me MY faults and what I had to do to keep him!!, but never making a proper commitment to leaving OW and generally trying to have his cake and eat it) but went back to OW in a matter of days. After a few weeks, I got sick of it, and used my anger to play him at his own game.
I pretended to still really want him back and to be desperate to forgive and forget if only he'd come back. In the meantime, I got him to bring round a load of things he'd taken, got back money he'd owed (well, 'borrowed' money from him to the tune of what he'd taken), found out exactly how much debt he had, got him to transfer over the ownership for the car (he was saying it was his, but we'd bought it for me to use as a family car, he had another car that wasn't so new), got hold of his payslips etc so he couldn't wangle out of maintenance payments, and sussed out his intentions all the while keeping him at arms length and pretending to desperately want him back.
It sounds manipulative, but he'd taken everything, and left me and new-born ds with nothing and the threat of no car and no maintenance. I knew he was still seeing OW all the time, and he wasn't staying round so I suspect he was still sleeping with her while he was sussing out whether to stay with me or not, so I felt no guilt.
Oh the look on his face when I had got everything I wanted off him, had the divorce papers in the post to his place of work, and I told him 'I'd rather chew off my own legs than get back with you'. Still makes me smile :)
Luckily he was the most disorganised person ever and didn't even seek legal advice.
Be strong, but don't let him know your plans, and keep your cards close to your chest until you have everything sorted.
Thinking of you