(FWIW; this account will become a throwaway.)
I will say that my wife's condition has "improved". Over a very long period of time.
When we were first married, I thought she was just a "difficult person" - but I was really really in love with her. That was really an intense relationship. Her issues were not really "that bad" at the time (I'm sure I'm doing some minimizing). It wasn't until right after our daughter was born that things really started to get difficult.
Her form of self-harm was not cutting. She abused laxatives. I thought maybe she was struggling with anorexia - no amount of reassurance from me would help her to understand that she was thin enough or pretty enough. Later, she confided to me that she did this, to feel the cramps in her belly. This made no sense to me for the longest time. I guess I kind of dismissed it. She said it was something she did as a teenager, but stopped after she got older. But I later found out that she was still doing it, into her 30's.
After a while, I started thinking she was depressed. By this time, I was certainly developing some codependency. My parents raised me to be mild-mannered and polite. Every time I stepped on someone's toes, I apologize. So every time my wife hurt; I was made to feel like I stepped on her toes. I kept the peace. :( I was maybe a bit depressed, and I was definitely having anxiety attacks. (for me, it was just kind of a weird horrifying "feeling" right where my heart is - I thought I was having a heart attack or something - I went to a cardiologist who said it was anxiety attacks. He just told me to stop drinking coffee.)
Later on, when my wife's behavior started to be more extreme. I'm not going to go into detail on the exact things she did. Just to say, it was very typical of other BPD things you read. Very twisted. No suicide attempts (as far as I know), but plenty of threats. And she blamed me for everything. I believed her. She was the Queen. She was the Witch.
We sought counseling, and the therapists we went to bought into her bullshit. Because I was accepting responsibility for all the problems.
Eventually, she accepted that she was "depressed", and got on psych meds (zoloft, and etc.) and this didn't really help much. Not that I saw. I kept asking her to go to therapy - and she did, once or twice, with mostly the same results. I believe she just lied to her therapists about what she was really doing.
During this period, I started seeing my own therapist, and ended up with a diagnosis of "Attachment Disorder" which, I guess, is a "step down" from PTSD (because there was no physical threat to my safety . . . that was what the psychologist said). I did a course of antidepressants for about a year. With therapy. Most of those symptoms abated eventually; but I still get triggered during stressful times. Knowing what it is is very helpful.
I decided that I would "stay for the kids" - and leave when they were 18. Having one parent with one foot out the door is not the best thing in the world for kids. But we were surrounded by other families where there was one parent. Those kids were all really messed up. Many of them were getting abused by step parents. I stayed to protect them (so they could get consistent parenting, and a stable childhood). I knew I couldn't get legal custody. Not with our current legal system. I didn't see leaving as an option until my kids were grown and on their own.
At some point about 15 years ago, I had read a description of BPD, that I thought was absolutely HER. (I know that I can't do a DIY diagnosis - it just fit).
She disagreed, and my own therapist "doesn't believe in this". She is still an excellent therapist, because she does help with important coping skills, and reassurance - she has been my lifeline to reality! But he just couldn't be any help in the way of BPD-specific coping strategies. It was all about helping me. (Which was important, by the way). Another very helpful thing was; I had conditioned myself to respond to my BPD wife so much, that I was losing the ability to even interact with normal people. I didn't even realize that, until I started to take care of myself more, and I noticed those skills coming back. I had become very awkward around other people. Walking on eggshells gets you through the day with your BPD SO. With normal people, not so much.
I don't have a lot of other choices as far as therapists goes: and I have tried a couple of others in my area, who were absolutely terrible. Sometimes they'd just chew up an entire hour talking about themselves.
Over time, I learned to be stronger, less codependent, and better at managing my relationship with my difficult wife. This was really important. After she got into her early 40's, things really did kind of even out. I think my wife changed from zoloft to seroquel at one point, but she never shared why that was. Well, she claimed it was for the insurance - but I don't quite believe that.