I feel terrible for not “being the mother she needs right now” and wonder if this really is a moment of change.
Hello and welcome to the family. You are certainly not alone and so many of us have faced the exact same obstacles.
Let me ask you, what type of mother do you think she needs right now?
From what you've shared, it sounds like you'd answer that by saying a mom that would let her stay with you, a mom that will fix her problems, a mom that would put up with her abuse, etc.
But is that the mom she actually needs?
What if she were a drug addict, begging for money to buy more drugs? Would the mom she needs be a person who would buy her drugs that could kill her, or a mom that would insist that she gets into rehab? Because in all honesty, that's exactly the same as your situation. The mom she needs is the one who will enforce healthy boundaries and hold her accountable for the mess she's created on her own.
Why? Because she has to want to make internal changes to actually start getting better. She has to take therapy and medication seriously. She has to work through this stuff, even though it's her greatest fear in life. Nothing changes until she's ready to make those active changes from within.
If a mom allows a person like that to sleep in their home while paying their bills and making everything the best it can be, then where's the need for that person to change? If things are bad, she can say that it's not her fault- it's your fault and her ex husband's fault. You guys did this to her....she's the poor blameless victim. That's the crux of the disorder, blaming everyone else so you don't have to take personal accountability.
I would encourage you to be the mom she actually needs...which is not going to be the mom she wants. This is not about wants though or what makes her feel better today, this is about baby steps towards lifelong changes that can turn all of this around and let her become a productive member of society.
My 27 year old BPD daughter was told last month by her therapist that they probably need to end therapy soon because it's not really needed any longer, she thinks my kid is stable and fully in remission. We couldn't imagine that three or four years ago, but she's put in the work and is coming out on the other side of this. There's still problems at times and she still grossly overreacts every now and then, but she now has the coping skills to work through it without the scorched earth approach.
Your kid can get there too, but it's 100% on her to put in the work. So be the mom that encourages that work and doesn't enable her to avoid it.