An example of this questioning could be as follows: "Why would you think I am mad at you?"
Questions are my favorite

I use validating questions with D20 a ton. I don't know why that didn't come to mind for handling this.
Maybe because excessive texting is a thing for her. She texted excessively with her dad when SO and I first started dating, and every now and then D20 will go through cycles where it comes back. SO has worked on this and done a really great job managing these particular boundaries and I admire how far he has come.
So I guess I've not asked questions in response because it feels like opening a door I'm not sure I want to walk through
I have to admit I hadn't thought that me reassuring him was a problem in itself. I can now see though perhaps I should be more guarded (?) - he wants his needs met and, by meeting that need, he then relies on my response.
This is good for me to think about. And goes along with this:
LNL - You've said that you're not in contact with her except for these texts. You've also told her you don't text a lot. To be honest, it sounds like she's wanting more from you than you're prepared to give. Do you want a relationship with her or prefer to self preserve and keep the distance? I totally understand if you don't!
I guess I have some unresolved feelings around this, especially because she is going to be living here again this summer, and last summer was so hard. It's twofold, in that there is a part of me allergic to any kind of excessive neediness. My boundaries are decent these days tho so that is less an issue than it once was, altho certainly there is room for improvement.
So that leaves this other thought... .I'm sorry this is long, I probably need to work this out. It isn't so much that I don't want a relationship with her, it's that I can see how me being ever so slightly detached is stabilizing for her. I do that by asking validating questions a lot, and acknowledge her competence at being an adult. Sometimes, her dad and sister treat her like she's an idiot, partly because she has a very helpless persona. But I think that helplessness might be a BPD response to what she thinks her dad wants... .she senses he likes to rescue, so she slips into something very childlike. Then she gets angry and/or disassociates, or becomes excessively needy, and sometimes all three at once.
If I am understanding things correctly, she feels separation stress (for example, being alone), then she feels anxiety about handling that stress (for example, being needy), and then she gets furious or feels self-loathing because she can't handle being alone or doing things for herself. My role seems to be validating the separation stress or whatever feeling she is having, and then reflecting back to her the confidence that she can do things on her own. That's the kind of reassurance I am fine with.
It's much easier to stay grounded when you're on the river bank and not in the rapids

And I think it's healthier for all of us as we try to figure this all out.
I am building on something I read and experienced. D20 and I had a couple of moments last summer where she was trying to sort something out (to do with her uBPD mother's anxiety and enmeshment). She asked my advice, and I did a mediocre job asking validating questions (encouraging her to be responsible for herself), and kinda validated her to the point she became very emotionally aroused, followed by cognitive distortions that were upsetting to her. It's hard to describe how this all went down, but at the end, she hugged me so tight it actually hurt and wouldn't let go. I feel she may be idealizing me, or expecting more from me than is healthy (for her, and for me).
In a book I read last summer (Loving Someone with BPD?) it said that sometimes it is best to not overstimulate a BPD loved one with too much intimacy. That it can be better to structure the relationship and limit engagement, since engagement and intimacy can be so triggering. That's what I have done with D20 -- make a plan to spend time with her every day. And then limit my engagement.
I have seen small miracles in our family unit when the people around her have better boundaries. I suppose this is one of those areas where I'm sensing that good boundaries matter, for her and for me.
She has sent cards that say "I think of you like a mom" and deep inside, I wonder if that is a feeling better expressed toward a therapist who can really help her. I am just trying to elevate skill levels to the point she is not totally lost at sea :