My 27-year-old daughter was diagnosed with BPD last December after she tried to commit suicide.
The attempt must have been so awful for her, and for you.
She is in therapy, but it is not dialectical therapy (which I have read is the most effective kind); she also doesn't seem to see the therapist very often. I also believe she is drinking again (I am unsure about whether she is taking pills).
There are other therapies that have been helpful, altho DBT seems to have been researched the most. Mentalization-Based Therapy is another (similar to DBT but designed to be less resource and time-intensive). There is also Transference-Focused Therapy (I think that's what it's called).
I would assume she is using, if only because the anxiety alone can be so intense that it is unbearable to get through the day without some kind of numbing to help cope.
What I really need advice about is how to get her to see her therapist more often and how I can get her to see that drinking is only a short-term solution to how to she might feel at a given moment.
Do you have a good relationship with her? A good place to start might be learning about validation. This can feel like a counter-intuitive skill. When my son was 8, he talked about not wanting to live. I would explain the million and one ways he was loved, what a smart and good kid he was, how funny and awesome -- this only made him feel worse. What he needed (I eventually learned), was for me to acknowledge and accept the pain he was in. As someone with codependent traits, I wanted to will him to feel better through rational explanations, whereas he needed to feel less emotionally alone.
If your D is feeling overwhelmed by daily living, not able to take care of herself much less her kids, telling her to see the therapist more, and at the same time not do the thing that is helping her cope, might create a cycle of shame out of which she can't pull herself. Validation may help prevent her from piling on the shame she is already experiencing.
I had to completely rethink my way of parenting. This new way is not intuitive, it's very slow and requires patience, and I have to surrender a lot of my dreams, which is not easy. None of the behavioral discipline techniques have ever worked for my son. They may get him to comply in the short-term, but they don't help with the long-term stuff. For that, I've had to learn new ways of communicating like validation and set (support, empathy, truth), and assert boundaries so that at least one of us is in a healthy place, and can be a source of strength when S15 needs me to be the emotional leader.