Hi khaki,
Welcome and hello

I'm sorry for what brings you here but glad you found the site. Is your daughter accepting her BPD diagnosis?
It's difficult to watch our kids react with such intensity to problems that can be solved. For people who struggle to regulate their emotions, there is a lot of psychological pain going on and not a whole lot of coping skills. My son has one of the biggest dysregulations he has had in almost year, and I tried something his psychiatrist recommended. I said, "You probably really need to get this anger out." Previously, I would be thinking "how can this kid be dysregulating over losing a video game?" The anger over the thing is not what he's angry about, though. The psychiatrist said the difference between shame and toxic shame is, "I screwed up" (shame) and "I am a screw-up" (toxic shame). So when my son does something wrong, he immediately goes to toxic shame "I am a screw-up." That's what the anger is about, his own confirmation that he's a bad person. :'(
There's a really good book called BPD in Adolescence by Blaise Aguirre (2nd edition) that has a very helpful section about cutting and self harm, and this site also has some information that might be useful:
BPD BEHAVIORS: Self injury and self harm
People can cut to feel something because they are so psychologically NUMB. *OR* they can cut because there is such a bottleneck of so much psychological pain that it RELEASES some of that pain. And maybe that goes back and forth from day to day in an individual. There are "better" things to do - like rub ice on your wrists, or pop a rubber band on them... .You can read more here... .
https://bpdfamily.com/message_board/index.php?topic=70493.0I recommend learning about validation and trying that with your daughter. It can have an immediate effect, although I seem to keep learning and learning and learning about it. That's essentially what the psychiatrist was trying to tell me, to validate my son's anger and try to ease off the gas when I see him overreacting to something. It's human nature to want to comfort our kids and tell them it's going to be ok. Kids with BPD, though, want to know that their feelings are valid, because they're in such emotional pain. They want to know that someone understands how they feel.