Help with an ex who doesn't want to let go

[ShouldaWouldaCoulda]

Ill try and summarise best I can.
6 year relationship split up for various different reasons, selfish, unappreciative, lazy, you get the gist.

I asked him to move out when I was five month pg with ds3 he obliged told me I held him back didn't need/want me would be better off without.
I asked him if he wanted to work things out because if not then there would be no going back for us.

Roll on 5 months later ds3 now 4 weeks old he wants to get back together, he's going to change, do whatever I want to make it work blah blah blah.

He has admitted to shagging a couple of other women so there's my line, he's crossed it, no going back as far as I am concerned.
Plus I'm happier on my own than i have been in a long time.
So I've told him all of this but still he still refuses to accept.

His replies vary from "I love you, ill do anything" to " ill never let you be with another man." Plus a few " if you don't do it for me, do it for the kids "

I've tried the gentle "sorry I still love you but just cant forgive you" approach (true)

and the hard line "your a cunt I deserve better fuck you" type responses
(true again)

Help please nothings working!

He doesn't have to accept it. What he does have to do is shut up and leave you alone or you'll have the police on his ass for harassment. ("I'll never let you be with another man" sounds horribly threatening.) If he wants a relationship with his DCs that's different, and to be encouraged in most circumstances, but not as a way to stay in your life.

You need to cut contact as much as possible and limit exchanges to the children and via text or email.

Don't respond to his messages, except perhaps with simply "it's over".