The diagnosis was extremely comforting to me, as I finally felt a sense of hope and a way forward. Unfortunately, she rejected the diagnosis, stopped talking to the therapist and has now placed him firmly in the "bad" person category--even though he did an incredible amount of good for us and is basically the reason we're together in the first place. I've tried to talk to her about seeing someone else/getting a second opinion but this inevitably leads to a nuclear argument (although I'm fairly used to those).
Finding out what is really going on in our relationships is truly a

moment. It's a relief in many ways and a place to turn all our attention too. All our desire to make it better can get focused on BPD. That's pretty natural. Like with most things in life, a moderate approach, especially in the beginning while feelings are still running high and we are still learning can pay off in the long run. Here is why.
Professionals do not recommend that you tell a loved one that you suspect that they have Borderline Personality Disorder. We may think that our loved one will be grateful to have the disorder targeted and will rush into therapy to conquer their demons, but this usually doesn't happen. Instead, this is difficult advice to receive and more likely to sound critical and shaming (e.g., you are defective) and incite defensiveness, and break down the relationship trust. It's not like a broken leg where the affliction is tangible, the cure is tangible, and the stigma nonexistent. While we are grateful to learn about the disorder and the pathways to recovery - for us the information is validating and represents a potential solution to our family problems- to the afflicted, it is shaming (you are defective), stigmatizing (mental illness in general, Borderline Personality Disorder specifically), and puts all the responsibility for the family problems on the loved one's shoulders
That is a quote from this link:
Getting a "Borderline" into Therapyand the highlights are mine. It's way to easy to incite defensiveness and break down the relationship trust. Once that trust has a dent in it, it's very hard to re-establish. For me, I found that being a 'partner' with my partner and working from the inside to effect change was definitely slower but also provided better results.
unfortunately progress takes a long time and consistent effort.
'ducks