Totally into the support group idea. I naively thought that my DP's ex was the only one. I'm both glad for my own sanity that she isn't, but sad for everyone, because this is a special kind of hell.
DP's ex has been described as difficult and unreasonable by the courts, but absolutely batshit would be equally applicable.
She does all the cancelling contact, not showing up, ridiculous and vitriolic communication, while at the same time has asked the CMS to assess my income for her maintenance. Our solicitor said that it's unlikely to change. He said she can ask whatever she wants but that ultimately it isn't my responsibility. My DC are my responsibility, while hers are not. DP does pay maintenance, never misses a payment, and we also buy all their clothes, toys, etc. For when they're here. Despite this, she insists that she has to supply items because we either don't have them - which we do, or because she doesn't like what we have - coat, not a uniform coat, just for messing around in, was apparently the 'wrong blue'.
She's also said she fully expects spousal maintenance - not yet divorced as she hasn't met a court order she won't break and he can't take the case out of child court until it's more settled. She said that she will be applying for spousal support, again based on my income. DP isn't working ATM, as he had to resign due to the extensive amount of time off for court dates, and the stress of the situation. We've explained that he's on benefits, it was a short marriage and she was the higher earner, but her response was that if DP wouldn't pay it, repeal will have to, because she needs the money.
As for why I stay? He's the best person I've ever met. I love him. And I'm big enough to admit, I won't give her the satisfaction of thinking that she has the power or control over our relationship. The other two reasons are bigger, and more important, but that last one is in there too on a much smaller scale.