I have been assaulted by her words and accusations for so long that I truly have PTSD from all the emergency phone calls, where I somehow end up the bad guy.
Indeed. I think I had a touch of PTSD from the emergency phone calls too. For a few years, I'd really tense up any time the phone rang at night (when the pwBPD in my life would tend to call with the emergency du jour). The mere fact of being "primed" for tension at the sound of the phone probably doesn't bode well for the conversation.
If you haven't realized by now, you should know that you are not to blame for all your daughter's problems, no matter how much she trys to convince you that you're the bad guy. That's classic BPD--blaming everyone else for her problems. When it comes to adult BPD children, I think that the chief object of blame tends to be the same-sex parent.
Have you noticed that your daughter has a victim mindset? Does she see herself as the victim of endless traumas? Does she twist facts into a convoluted victim narrative? That's her maladjusted way of coping with her general negativity and emotional overwhelm. She feels upset, inferior, confused, tired, aggrieved, treated unfairly, ashamed, entitled, impatient. She thinks everyone else has it so easy, and she's insanely jealous. Meanwhile she wants all sorts of adult privileges and freedoms, but she doesn't want any of the associated responsibilities or work to earn them. When she's dealing with these negative feelings, she'll reach out to you, probably to give them outlet and blame you in the process. Does that sound about right? I've written here many times that I think the victim mindset is the worst part of BPD, because it renders your daughter feeling helpless and powerless over her life. How miserable that must make her feel.
I think that when your daughter called you, she was upset about starting the IOP. She probably didn't want to do it, because she'd rather do something else, even if it's just lying in bed and watching videos. Or she was scared, intimidated, a little confused about the process, uncertain about what she was facing, how it would be paid for, the rules she'd have to follow even though she didn't want to. Maybe she was just tired--the notion of getting up early, packing up and finding her way to the center was overwhelming. Whatever the reason, she calls you, she becomes semi-unhinged, and she stirs up drama, possibly picking a fight. Why? My guess is she does that so she can find a reason to blame YOU, to hate YOU. She talked about "cancelling" the IOP. My guess is that your daughter is looking for an excuse to quit, and to blame it on you, even though she knows she ought complete the program. Deep down she knows she's a quitter, and so she's really conflicted. This induces in her feelings of shame, failure, frustration. Wanting to hate and blame you so much, it's basically code for how much she blames and hates herself.
If you want to know what's bugging your daughter, I think you listen for the feelings behind her rage/accusations/insults/objections. She's ruminating so much about these things that when she's upset, they emerge as projections onto you. If you understand that, it might be easier for you not to take her verbal assaults so personally.
How do you support your daughter these days? I think you try to be as calm as possible, while projecting cautious optimism. Look, when my pwBPD was getting intensive treatment, I think she warmed to the notion of getting professional help. Subtle messaging like, "It's great you're taking care of yourself," or "It looks like you picked an excellent program," or "They are professionals, they know how to help," or "I'm proud you're prioritizing your health right now, that's really mature of you." You might add, "This must be hard for you, but it's just temporary," or maybe, "I imagine you'd rather be doing something else, but working on yourself is a priority right now, everything else can wait." Or maybe, "Doctors help people overcome trauma all the time, they know how to help." If she tries to dredge up the negative past, you refocus on the present: "The important thing right now is that you're getting some help to deal with all this." If she's overwhelmed by the future, you might say, "Let's focus on right now, just the next few weeks, and you'll figure the rest out later."
I think the upside of the IOP is that you are relieved from "fixing" your daughter's problems. Let the therapists / doctors take on that role . . . they have the benefit of no prior emotional baggage. Since they are "professionals," there's a chance that your daughter will listen to them. I think what you try to do is gently nudge your daughter to comply. In other words, you are her ally in following doctors' orders. In my opinion that's the short-term focus.