I realize it's not my responsibility for fixing her but I feel like after 20 years I have to at least try to get her the help she needs. If she won't then that's on her but at least I will have tried.
Stop for a moment to ponder... haven't you been trying all along? And more recently you've learned about Personality Disorders, more specifically Borderline PD? And now, despite you being more informed and focused in your efforts to help her, she's even more oppositional?
Sadly, this is not uncommon. BPD is a disorder most impacting to close relationships. Others on the periphery may notice "something off" but the closest ones bear the brunt. Unfortunately, Denial and Blame Shifting are strong traits in people with BPD (pwBPD). Combine all that with another observed trait, that the history of your close relationship is just too much emotional baggage for many with BPD to truly listen to those close who are trying to help.
Years ago I read a comment by another member and that explanation opened my eyes. In short, BPD is a disorder impacting close relationships the most. Others on the periphery may notice something "off" but it's not a big thing in their lives and they don't spend a lot of private time where the pwBPD feels free to "let it all hang out". But we did get the full brunt of the poor behaviors, and you do too.
Why couldn't we help? Because we shared too much history with them and they couldn't get past their perceived emotional baggage of the dysfunctional relationship in order to truly listen.
It's not for nothing that all the professionals recommend intensive therapy by therapists who have no emotional attachment to their patients.
If she will not listen to anyone, accept that. Maybe some year in the future she may decide to seek meaningful help, but odds are you're not the one to do it. She would have to want to fix herself.
Normally I would emphasize that your priorities are to protect yourself and your parenting. However, you haven't mentioned having minor children so perhaps you only have to protect yourself, especially legally and from false allegations. As has been mentioned in years past, "If it has been threatened or even just contemplated, it
will happen, given enough time." There is almost no limit to what your spouse may do or attempt.