She left me four months ago. She thought I was going to leave her so she put a back up plan in motion and found someone to take my place. She will tell me she doesn't love him and that she loves me and wants me. But she has had plenty of opportunities to tell him and she hasn't. He moved back here for her and quit his job to be here for her... .
I was never going to leave. In fact I wanted to move forward. Nothing has been easy but we have always fought through it.
Good mental health is hard. It takes a great deal of strength in times like this.
You seem to have a good handle on all the pieces.
~ she left because she prefers the honeymoon stage of a relationship over the building a life together part of a relationship - instant gratification vs work and self examination
~ sounds like she is telling the other guy "soul mate" kinds of things and his adoration and actions make her feel "perfect" rather than "defective"
~ she never grieved what she lost from your relationship with her (and there was a lot) and is now feeling that loss
~ a little reality or a little self doubt may be creeping in on the honeymoon and you are a great source of validation and reassurance for her.
This is BPD emotional turmoil.  :)oes she love him?  :)oes she love you?  :)oes she even know? Is the emotional maturity and strength there to sort this out in a responsible way or is she so overwhelmed she can't think past her own needs right now?
The first question for you is, regardless of where this turns in the next act, does she have the emotional maturity and skills to be a life partner?
The second question for you is, do you have emotional maturity and strength to navigate this better than her? Clearly you are struggling with some abandonment anxiety (as we all her do) - can you rise above it so that your emotional satisfaction for making a hard and mature decision is not flooded out by abandonment anxiety.
Good mental health is hard. It takes a great deal of strength in times like this.