My exdw was BPD. She had an abusive childhood and as others have described, morphed into a paranoid, manipulative and EA partner. I left as a result of discovering a long term affair, we tried to deal with it but she followed the Script and was too stubborn to change. Lots of other stuff, blaming, projection etc.
When I said I was leaving she started (or rather, stepped up) the suicide threats. Elaborate things like buying lengths of hosepipe and tape, stockpiling paracetamol, trying to jump out of the car on the motorway etc. Leaving notes and disappearing for hours. It was terrifying and traumatic.
Lots of other things too like simply pretending conversations hadn't happened, forcing me to restart incredibly difficult and painful talks. In the background she was briefing friends and family against me, painting me as the abuser etc. I went on antidepressants to control the panic attacks.
It took me around 6 months to realise I should just leave. I didn't need her permission, understanding or agreement. It was hard but we all survived.
Here's what I did: I de-escalated all arguments, responded factually and minimally to texts. I put phone calls through to voicemail then picked up the discussions by text or email. Attempts to send messages via our DD were redirected back to text. Attempts to manipulate DD were ignored - I focused on simply having quality experiences with her and she learned that I wouldn't enter into blame discussions, and I spoke positively (but minimally) of her DM. This kept her out of as much as possible and she learned to cope.
Now we have both moved on. DD is a remarkably stable teen and we even parent well together. It's taken a few years but we got there.
One comment on BPD. It is treatable but often fails because BPD sufferers are frequently in denial. They are also frequently very clever and can manipulate those treating them into thinking it's working. It's a very difficult condition for a relationship.