Remember OP, you didn't cause this; you can't change this and you can't cure it.
In any real relationship you both have a say; instead he's given you no opportunity to work to make this better but has unilaterally decided to bog off on his own (ridiculous) journey and now he will have to live with the long term consequences and the regret.
He has changed, he will never be the same person again and this breakup was HIS CHOICE!
He will try to rewrite history and blame you. He will gaslight and In your low moments you will probably go round in circles haunted with fears of how this must somehow be your fault - NO NO NO! (1000 times NO.)
You can allow yourself that you might have contributed to the situation (given that you are both in this relationship) BUT that is NOT the same as you causing or creating it.
Trying to change yourself to make yourself more like you think he wants you is utterly pointless. He'll be making it up as he goes along to justify his shitty actions and probably won't even remember what he's said. Claiming that you were unhappy is a fairly typical projection and trying to sort it is a pure waste of precious cuddle time with your lovely children.
You won't want to hear this but it is highly probable that he has had his head turned by some silly female. The absolute prat.
You sound brilliant and so strong OP, you are handling this so well and all of MN is behind you. He doesn't stand a chance!! Keep telling yourself no buts, you have done nothing wrong and can hold your head up high
Detach as much as you can and don't give him an inch. Don't agree to any of his demands just say you need time to think about things and you'll get back to him. And quietly harness any anger - it is really energising. He'll be put out that you are getting on with your life and not weeping and wailing. Keep it that way. (Of course you'll have spells of said weeping and wailing but he doesn't have to know about them.) He is no longer your friend and he doesn't deserve you.
They say that the lucky ones are the ones whose husbands don't come back...
As the sole sane grown up you're the boss now. Take no shit from him.
Big hugs to you x
On a lighter note I must just leave you with a dire warning ... you don't say how old he is but beware that he might start to wear the midlife crisis man's uniform at an inappropriate age... (i.e. Superdry!) 
There are hundreds of them wandering up and down a high street near you and not hard to spot!!!! 
