Hi Sunset,
Welcome and hello

I'm glad you found the site, and sorry for the sadness and pain that brings you here.
I imagine your D31 is grieving her new condition and the uncertainty about the tests is intensifying any anxiety she may feel. Not to mention that moving back home with a parent can cause a child to regress, just that adjustment alone. We have an independent, lovely SD23 living with us while she goes to graduate school (her mom is BPD), and it was quite a tailspin to watch her regress to middle school behaviors. And she's not even BPD.
When your daughter feels despair, how do you respond?
Maybe we can share with you some of the (non intuitive) communication skills that can help prevent things from getting worse.
You're not alone, Sunset. And I agree with incadove that there are new evidence-based skills that can be applied to our loved ones and their distress.
First we have to make sure we take care of ourselves so we have the emotional strength to support our loved ones.
LnL