I have been where you are. I called time in desperation about 7 years into my previously mentioned “12 Years With A Narcissist” stint. It had taken me far too long because in my head I had to be able to get him to agree I was right to end it, because I was so scared of him telling me I was being silly, disagreeing with me, and somehow I felt like I had to justify myself. He cried. He said all the right things. He agreed to go to Relate with me. He agreed to come to bed at the same time as me most nights and not sit up on the internet telling everyone they were wrong. He agreed to be nicer to my mum.
After the initial heady burst of relief, and then the sensation that together we were unbeatable (nothing quite brings you together like this sort of catalyst), I felt…trapped. Even worse than before. I had bloody done it, I had said it, and then I had gone back on it and it would be even harder than before.
It didn’t last very long before we were back where we’d been, but I felt even more stuck. And since it hadn’t worked once, why would it work if I tried to leave again? Eventually, he did what your DH has done - flicked a switch and treated me dreadfully for months, making everything my fault (taking his hand when we were out for a walk was “clingy”, for example), ruining my graduation ceremony for my second degree (aged 39!) and spoiling the day for me, my mum and my best friend by making it very obvious he didn’t want to be there, etc. Then he said he “wasn’t sure he wanted to be with me and was making his mind up”. I spent two weeks of hell waiting to hear my fate, during which time he wanted to have sex, drive me to the station, host a dinner party, and “go on as normal”.
Long story short, he then called time. Said there was no-one else. I found out three months later there was, and had been, for quite some time. (He stayed in the house for months because it suited him while I bought him out, I had to play that very carefully).
Anyway. Moral of the story is wait for the cold light of day before giving him another chance. If he was really serious about you, your family, your feelings, he wouldn’t be slyly telling you how hard he’d fight for custody - that’s just to scare you and put you off. The reality is the court will do what’s best for the children and he will not paint himself in a good light. Hang in there, and remember: tomorrow is likely to be better than today; next week better than this week; and so on. One breath at a time.