Hi Dorkus, glad you found us
Fifteen is young and you're right, there can be a lot going on. Has a professional suggested that diagnosis? Has she been assessed for other similar-presenting issues?
Makes sense that whatever the label (or not-label), that doesn't change that what's going on with her is beyond difficult. If you're here, you've been through it.
So it sounds like you're "the bad one" in her life right now? Is her dad in her life, and/or siblings? What's her relationship like with them and/or other family members?
You may have run across this already -- Dr Blaise Aguirre studies and treats BPD in adolescents. I haven't read the book so not sure if he "leans" towards labels/diagnoses in teens or not, but it's worth a try. Whether or not he's in the "do diagnose" or "don't diagnose" camp, I believe the book may have valuable suggestions for, like you're wondering, how to be strong and stable and have tools to lean on as your daughter rides huge waves of emotionality.
Check out the link and book, and let us know what you think:
https://bpdfamily.com/message_board/index.php?topic=65571.0-kells76