Hi there, I'm going through something very similar. I'd been bending to my ex's whim for so many years to keep the peace, and carried that on after the split. I would go into a spin just from receiving a text or email from him. My ex is so used to being in full control and is obsessed by money (and hording it all for himself). I have also learned that he lies, a lot, so cannot be trusted, on any subject. Mine is currently trying to say his business has suddenly become unviable too, it's common.
Are you having counselling? Request it via your GP if you need to or self-refer for NHS counselling. I was on the waiting list for 5 months but it was worth the wait. My counsellor recommended 2 books and they have completely changed my mindset, please order them:
Nasty People by Jay Carter and It's Not You by Dr Ramani Durvasula. Nasty People has reset my brain.
This is what has helped me:
Off-loading to the Counsellor who is very practical and experienced with people leaving abusive/controlling relationships. You need to remove your ex from the 'centre' of your world. You're not in a relationship with him anymore. Keep telling yourself that.
Those two books above.
Unless it's about the logistics of seeing the children or an emergency to do with the children, do not speak to/communicate or be in a room with him. He's a liar and abuser, remember that, and he will lie to everyone to stay in control/hide money/win an argument. Only respond to him if it's in writing and it's to do with the children (and requires an answer). Everything else via the solicitor.
Swearing. Lol.
Stop caring (it's hard) about what he's telling people about you/turning the kids against him. As above, he will lie to keep in control or push his narrative. YOU know the truth. This is hard and will take time (I'm not there yet).
Ignore any tirades or any attempts to pull you back in. He will love the drama of sparing with you, or getting you in a tizz, regardless of the subject (money/custody/CMS/anything to get your engaged). Ignore. Do not give him a response at all.
Remind yourself that you and your children are the only things that matter and you are a team. Your ex is a nobody, keep telling yourself that. I'm only 6 months down the line and me and the children are doing okay, and it will get better.
I'm happy to chat if you want to DM me.