Haven't read your previous thread so apologies if I've missed something, but you said here:
"We had the original argument three weeks ago on Wednesday evening and the Wednesday morning we were our doing usual family things, Christmas shopping, bite to eat etc and he himself admitted that in the morning he was happy, he wanted the relationship and he wasn't having any doubts" why were you having conversations in the morning about whether he was having doubts about the relationship? Surely that is an indication that this hasn't actually come out of nowhere but has been building for some time but that it all came to a head on the Wednesday evening?
Added to which, I agree with @offred you're not doing anyone, not least yourself or your kids, any favours by pandering to this. And it is pandering, as hard as that sounds.
I understand that it's apparently down to MH problems. But even MH problems can destroy a relationship. The person suffering needs to want to get help, and clearly he doesn't.
You now have to rebuild your own life with your DC's, learn to live as a single parent, show him that he and he alone is responsible for himself, and then, and only then can you be genuinely in a place to rebuild your relationship if that's what he wants.
But for now you need to assume that actually, that's not what he wants, and neither can you take responsibility for his failings both as a partner and as a parent.
So tell him that you need to work out a visitation schedule for the DC's, assuming his MH means he is safe to be with the DC's. If not then I would tell him that as you don't feel he's safe to be with the DC's alone he needs to get help to be there.
Reality here is that he is capable of being responsible for things, but while you and his mother are pandering to his poor me attitude he's not having to take responsibility.
But he managed to move out independently and tell you that the marriage was over? He's managed to go out and buy a Harley with (presumably family) money, now he needs to take responsibility for his health. If he's not prepared to do that then he and he alone is responsible for how things turn out. And if that means you move on and file for divorce then so be it.