Happy new year, ConfusedinCA!
I'm sorry for the grief that brings you here, and glad you found the site.
My partner's daughter (19) has a similar history to your daughter. Diagnosed bipolar during her teens, though I am certain she is BPD, which is apparently a common co-morbid diagnosis. The correct diagnosis probably matters most when sufferers are looking for clues to their inner turmoil and wanted to find treatment and solutions. Since your daughter is not ready, at least the diagnosis can give
you that understanding. Skilled family members can make a big difference. A lot of BPD behaviors are experienced (perhaps triggered) in close intimate relationships, so knowing some relational or communication skills can help -- including validation and asserting boundaries, as you mention.
Validation is a great skill. My favorite version is
asking validating questions, which puts accountability back on their shoulders, where it belongs, and gives them a sense of adequacy.
I also find it is helpful to dig deeper into validation skills that address accountability and boundaries, and books like Loving Someone with BPD by Shari Manning are good, as well as Valerie Porr's book (title escapes me) are good for that. Things like learning that having a calm, neutral face when someone is in a state of heightened emotional arousal can actually be invalidating, leading to more problem behavior instead of less. I've had to learn to be more expressive with my face. More this

than this

so to speak.
I also learned that sometimes, limiting exposure is healthy for your BPD loved one. We can be triggers for them, and sometimes, if they are prone to dysregulating over certain topics, it's best to redirect the conversation or take a physical break so they can reset.
What are some of the ways your daughter is struggling most? How does she respond when you say no to her or deny her something? What boundaries does she push the most?
Does she live with you? How often do you see her?
LnL