Oh dear Heather
I have been in your exact position. The only way to stop his behaviour, is to really toughen up.
has said his affair is irrelevant as it is in the past and that I broke up our family as I refused to take him back and moved on with someone else
Yes, my ExH said this to me as well, despite him playing away on several occasions, apparently, I was the bad guy, for leaving him and breaking up a family. His parents agreed, as did his siblings, and none of them ever spoke to me again!! After a 20 year relationship! Just fucking bizarre.
My ExH also tried to push my buttons, manipulate me after the split, and also didn't want to may child maintenance, as he thought he shouldn't have to "fund my lifestyle" 
He tried to get more out of the settlement than he was due, claiming poverty (he's a high earner).
I pursued CM, and he did pay, but would sometimes stop paying, so that I would need to ask him to pay (manipulator, you see?). So, I went through CMS, and it came to light he'd lied about his salary and under paid. He now has to pay me for this (yey).
I also did not accept his poverty claims, and got my fair dues from our split, so glad that I did, because only a few months later he was spending ££££ on his new GF (again, it was all lies).
He also told everyone, that my now DH was a person I'd had an affair with before we split up. Another lie, I was just lucky and met DH only 2 months after I left ExH.
He will keep making demands on you, because you let him. My ExH demands would slowly ramp up, until I lost it and screamed at him, and then he would go back in to his box for a while.
We finally got on to terms that weren't too bad, and then the very day the youngest went to Uni, he went NC with me. Contact was only ever about the kids anyway, but was still needed to a certain degree, as we still financially support them. Another form of control.
- He is not your friend anymore
- He does not have your best interests at heart
- He doesn't want your new relationship to blossom
He doesn't want you to be happy (you lost him* how could you be?)
- He will screw you financially, if he can get away with it
- Do not engage in convo with him, unless by e-mail/text
- Ensure shit hot lawyer get you the best deal possible.
- Introduce your new man, when you feel the time is right. He does not get a say.
- If he suggests things that don't suit you, say firmly "that doesn't work for me. I am going to do it this way instead" Then disengage.
- Raise a case with CMS asap, as they can only pursue payments from the date you open a case with them. It's £20, but worth every penny. My ExH is having to pay me arrears now, for the years he under paid. I could never have extracted this from him, without the CMS going down the legal route for me.