OP - I understand. I contemplated walking away from one of my children because my ex was obsessed with him and I thought he would leave me and the other two alone if I did. I just wanted him to stop.
There are no easy options but I am sure you wouldn’t live peacefully if he gets what he wants. He might remove himself physically from your life but your children would be forever on your mind.
Have you done the Freedom Programme? It would help if you haven’t. Revisit it if you have. There are aspects of it that I go back to when I need to. It helps.
Don’t add to the drama. Xmas is a day. You can celebrate it in July if you want to. I have lost count of the Mother’s Days I haven’t spent with my children. It really doesn’t bother me. We just do it a different day. If he can’t hurt you with this stuff, it loses it’s power and more importantly, he loses his power.
I don’t think you would lose in court but I don’t think he would win either. He is manipulative and at some level neglectful if he keeps losing one of the children but unless some harm comes to them in his care, you will struggle to get a court to stop contact. On the other hand, your children will be able to discuss with a CAFCASS Officer how they experience their time with their dad and that might be enough to restrict access or make it supervised. Your children will be aware of what is happening and how it makes them feel and may well articulate that well to a stranger. It is a gamble, however, and you need to be strong if you are going to represent yourself.
You may find a call to Women’s Aid helpful for advice and support. There is also a free legal helpline run by Rights of Women. Wikivorce is an excellent source of information on all things legal.
As a final thought, your children will judge you on your actions. They need one of their parents to put them first and bite their tongue. I found it easiest to generally let the ex mess me about with contact with a nod and smile. I put huge effort into having watertight childcare arrangements and just waving them off with a nod and smile. I would say from the age of 13, my eldest showed signs of understanding. At 16 he has a positive relationship with dad but he knows he can talk to me and knows we both see him for what he is. He is angry his dad sees him as an extension of me to be manipulated and lied to but he is grateful that he has worked that out for himself. He accepts his dad and laughs to me about all the inconsistencies and stupidity he demonstrates. I think this is how it should be - his dad is an idiot but me ramming it home would, potentially, have sent the children into his arms, Instead, they know where they are safe and which of us has genuinely made sacrifices.
It is hard. It is not how you planned your life would pan out. But you can live it and manage it and build your own life as well. Be kind to yourself, build your support systems and let that asshole go right over your head. It is possible and you can do it.