I want to share some resources (and a little bit of my experience) about being in a relationship with someone who has Borderline Personality Disorder whether the relationship is current or past. My mother, I'm 99% sure, had BPD and I had a relationship for several years with someone who had BPD. Borderline Personality Disorder doesn't seem to be mentioned that often on relationships, but I'll often read posts where it appears like it could be a factor. Not all abusers have BPD and not all people with BPD are abusive to people they are close to, but they frequently are. The advice on relationships can be really brilliant and can be appropriate for people in relationships with someone with BPD, but I want to encourage people in who know/think that they are/were in a relationship with someone with BPD to get some specialist information and support from people who know BPD inside and out in addition to posting here.
Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder is a really good book. It was recommended to me by friend and it was like every single piece of the puzzle clicked into place. My ex partner had tried to explain a lot of the things in the book, but I never really understood them (or so a whole host of other things) until I read it and also the excellent BPDFamily.com message boards, especially this page. The revised edition of I Hate You--Don't Leave Me: Understanding the Borderline Personality is also supposed to be extremely good.
I won't repeat all the excellent advice that's contained in the book(s) and links above, which can be summarised as: 1. You must look after yourself, get help and support, set boundaries, leave if you need to & 2. You didn't cause it, you can't cure it, you can't control it (The 3 C's of Al-anon). You can only change you.
But I do want to advise one thing that isn't often mentioned when discussing people who have been abusive to you and that is to be compassionate. I know that some people will find that a contentious or very upsetting statement, but I believe it helps in getting over the immense hurt and the headfuckery that is BPD. People with Borderline Personality Disorder are in emotional pain all the time. Their awful behaviour is an attempt to cope with or deflect immense pain - that doesn't make it OK and you don't have to put up with it, but it really does help to feel calm and detach when you fully understand what it's like for them and understand that it's not about you, that they didn't ask for this either.
I really hope this helps even one person. Being in a relationship with someone who has BPD is often very isolating and it can feel like no-one could understand the crazy that your life is/was, but getting support from people who know exactly what it is like is an immense relief.