You talked with such love and respect for this girl. I can see how much you care of her. It's very good. This is love but for your own well-being...you also need to have a little bit of selfiness. I understand you need to look for an explanation. It's a real shock to see how a person changes so much from one day to another. But, it happens more often than it's to wish.
I wouldn't even put this person a label of a PD. There are normal people who are like that.

Agree that the relationship forum is the best to talk about this. I can't tell you a lot. I'm sure I will be wrong or at least, partially wrong. Only that look for the answer on your own.
If you look at back, I'm sure you will find red flags. Can you see them? It's a question for you.
I know you need to find an explanation to mourn.