Hi Slash! Welcome to the family!
I know what you mean about having difficulty in wrapping your head around it. I've known about BPD (and that my husband might have it) for a little over a year and I still can't quite get a handle on it sometimes.
Trust is a HUGE thing with my H too. It's wrapped up in his fear of abandonment, I think.
She has told me that I need to figure out a way to fix it, and restore that trust, however, I don't know what to do.
This is the sort of thing my H used to say when he was dysregulating. He would get upset about something and demand I fix it. At his core, he was in pain. He didn't know how to manage that pain. So, he lashed out at me, trying to throw the pain at me so that I would make it better. He was desperate. The problem is, I didn't know how to fix it -- or really could not fix it. Because we can't. We can't "fix" the feelings of others.
I used to run around like a chicken with its head cut off trying to "fix" for him. It never worked and probably just made me look crazy and unstable. Now, I practice disengaging (to an extent). His feelings are his. Mine are mine. I work at not getting sucked in by his dysregulations. It's not easy and I don't have it down pat yet, but I'm working on it.
Have you asked her what she wants you to do to restore that trust?
Also, do you know much about SET (Support Empathy Truth) statements? We have a workshop on it here:
https://bpdfamily.com/message_board/index.php?topic=143695.0This communication style might prove useful to you when she's making demands and accusations.
What do you think?