Wow, a breakthrough!
How did you/DH respond when SD14 shared how she felt?
I'm looking for thoughts on tools and healthy coping mechanisms.
Last summer, SD20 had a dysregulation about her mom being insensitive to her needs, and shared her frustration with me. I validated how it would be frustrating to anyone, and asked her if there was a specific outcome she wanted, and what skills had she tried in order to achieve that outcome.
Letting SD14 know that there are skills might give her a sense of agency, and it could also introduce her to the idea that she may need to experiment and practice to see what works (while demonstrating that she is actually taking care of herself).
I also make a point with SD20 to point out when she is taking good care of herself, instead of on the outcome, so on some level she is aware that the goal is to take care of herself, even though her attention may seem to be focused on her mom.
The two best books I found for learning BPD communication skills are Loving Someone with BPD by Shari Manning, and Overcoming BPD by Valerie Porr. I wouldn't necessarily suggest them to SD14 -- they would be more helpful to you as you support SD14, to give her guidance. If she can identify one aggravating scenario she wants to focus on, you could look through the book to find a similar scenario and see if she wants to try communicating (usually using specific phrases) with her mom to see if the results are any different.
I also found the phrases in I Don't Have to Make Things All Better by the Lundstroms to be super helpful. Sometimes SO ties himself ina knot trying to fix BPD type issues and I toss out a validating question he could ask instead, so that his BPD person is held responsible for fixing her own problem instead of him.
I imagine that will be a big thing for SD14, to have language (and a conceptual understanding) that her mom is ultimately responsible for fixing her own problems. SD14 can express empathy and be engaged, without taking on a fixer/rescuer role.
Last thought -- S16 was 14 when he was struggling with some texts from his uBPD dad. I suggested he share his concerns with the T, who simply said, "That sounds like an adult problem to solve, not a S14 problem to solve." It gave S16 so much relief to be removed from the middle with that simple comment.
Because a lot of the time, it does seem like the big injury is when our kids have to solve problems for their BPD parent, reversing the natural parent-child role. Knowing when to let them off the hook, and helping them communicate skillfully is kinda tricky and mostly I bumble through it, but at least there are some things that seem to stick and at least in general improve the overall skill level, mine included.