I don't know if I have any helpful advice, but I can at least offer empathy.
I am in a similar situation, only my exH had uBPD. My fiancee has not one, but two exes (one kid with each) who has uBPD. On a 'crazy' scale of 1-10, my SD's mother is probably an 8, but my SS's mother is a 35.
I have struggled in the two years we've been together, as I am also triggered heavily by their antics. My SD's mother is easier to deal with, as she lives 2.5 hours away. The only time she gets under my skin is when we (thankfully rarely!) are in the same room together, as she completely and totally ignores me. We have primary custody of my SD, so it's annoying to say the least that she's not able to be civil with me as I'm raising her child 90% of the time. But other than being overly friendly sometimes with my fiancee it's not too bad.
SS's mother however is a whole other beast. She is truly and utterly delusional. She constantly harasses us, insults me and him, turns my SS against his father, threatens all kinds of things, and generally just goes on rampages at fairly regular intervals. As this kind of verbal abuse and behavior reminds me of my exH, it's difficult sometimes to not go back into my shell and my old defense mechanisms.
A couple of ideas, and maybe you've already tried them but here is what has helped for us:
- My fiancee does his best to filter the noise. I'd say 90% of what she spews is empty threats and just hateful words. There's really no reason I need to hear every new attack. Unless it's something that could affect the visitation schedule, something so extreme he wants me to help him log it, or something that may indicate we need to get ready for another legal battle, he just doesn't tell me. This is hard for me, as with my background I have this false belief that if I know every little detail of the situation I can somehow control it or prevent bad things from happening. But the truth is they don't change, it's the same old song and dance, and it's not worth the stress on me. The less stressed I am, the healthier I am (which is less stress for him), and the better I can help with daily support for him and the kids.
- IF he has to tell me something, and IF I can't control my upset then I have to counter my responses with positive thoughts. I try and remind myself that she's 'winning' because I've allowed her ridiculous behavior to affect me, and that she's managed to cause a wedge between me and my fiancee. This normally leads to me not wanting to grant her any more power, especially from me where she has absolutely no hold. Sometimes I'm really successful with this, sometimes not but it's better for me to try than to just let myself spiral down into the anger and frustration.
- If you haven't read about C-PTSD or any of Pete Walker's book I would recommend it. I just started one but some of my trigger responses are described perfectly. I think it will be a useful tool as I get father into it.
- I also have absolutely no contact with either of them, outside of the extremely rare school event for SD. Even at those events I've stopped trying to speak to her or even be civil. With SS's mom I had to threaten legal action, and have blocked numerous phone numbers due to her harassment. At the end of the day, there is really NO reason you need to have any contact with them whatsoever. You are not in a normal situation, you guys are never going to be BFF's, they are never going to accept you as a co-parent to their child. So drop the delusion and protect yourself!!
I really think it's a long journey. We learn to set boundaries with our FOO or exes, and then as we encounter different types of PD in our lives it's another learning process. I remind myself that it isn't forever - even though they share children, the kids WILL get older, and he will be able to go full NC one day vice the LC he has now. That helps some too.
Best of luck to you, and I feel your pain!!!