I was going through all kinds of scenarios like this too. They left me mostly confused But review of some of the key ones is necessary and healthy I think. Well done on the efforts to process what you've gone through Larmoyant

I am ashamed to say that I was very relieved.
It's normal for us to feel some shame at specific incidents when we feel responsible for the actions of others:) The responsibility for specific persons and specific actions seems to me another matter.
We’d gone to visit my sister who was having her gall stones removed. He had not met her yet (2 years into the relationship!) as I felt the need to protect her and myself from his negativity and was afraid he’d run her down like he had my other sister.
I think this is a good call on your part.
My sister had been scheduled for surgery and had it cancelled several times due to staff shortages. We didn’t stay long and he was very attentive and seemed very concerned for her.
This could be anything.
After the visit he insisted on making a complaint and we went to see administration about it. He controlled himself, but I could see the tension rising.
Okay. It's believed that for some pwBPDs, behaviours manifest only in specific relationships.
30 minutes later in the car he began to snap at me.
Back in the car I braced myself for his rage, but to my surprise instead he told me what had happened with the ‘unprofessional’ nurse and was defending his actions and wanted me to validate them.
I charted my ex's dysregulations. Mine had a pattern, then it changed, then it changed again. So no pattern

I've heard some pwBPD have more predictable behaviour and triggering stimuli.
What could be more interesting to you is, how did you respond in the car? To encourage you, I'll share what I did--hopefully you did better. Sometimes in times like this, I inappropriately caretook, smothered, and mollycoddled my ex like a 12-year-old. She loved it. Bathed her in milk. Soaked her in honey. Super enabler. A+. Even that can be a super unhealthy response for a 12-year-old. Not really the picture of a healthy relationship:)
Is there any potential learning you can see or feel here Larmoyant? Anything that comes to mind. Relationships, people, anything for yourself too?