heartbroken, I'm so sorry you're struggling with these ruminations. I know you're in a difficult situation with your dBPDh. My heart goes out to you.
You're absolutely right - healing and working on ourselves is a process, not an event. You've come so far with yourself.
I think it's perfectly natural to have thoughts like that -
are they capable of giving someone else the love and intimacy they couldn't give to us? What is the replacement like? What is the relationship like? This is someone you devoted a lot of time, energy, and love to - of course letting go isn't easy. These obsessive thoughts keep the person "alive" for us, and for the more masochistic of us, can be effective forms of self-flagellation.
But deep down, underneath that obsessive narrative that has you questioning yourself and your reality - you know the truth. You've known him for 28 years. You know his patterns, both before and after his therapy.
It seems more likely that, at only 2 months in, they are both experiencing the intense idealization phase of the relationship.
As to how you "should" feel about her... .well, there is no true right or wrong, there is only what feels right for you. Just remember to be kind to yourself - don't compare, because that only leads to unhappiness.
I agree that processing and exploring our deep thoughts and ruminations helps us get through the muck and mire and find ourselves.

What are these particular questions telling you about yourself and your feelings?