Something to keep in mind is that beneath the blaming and threats to leave, is a deep reservoir of shame and self loathing in individuals with BPD. They may quite successfully cover it up much of the time and project that negativity upon others, and perhaps due to the isolation of Covid, you’ve become the convenient enemy.
It’s very good that he’s seeing a therapist and that you are too, as it’s very stressful to live with a person with BPD who is acting out their discomfort.
You ask for an example of a successful marriage. Well, I’ve divorced one BPD husband and am currently married to another one (family of origin predisposed me to thinking that people with BPD are *normal*

) and we are doing well, having been through a rough patch a few years ago.
What makes a difference from my experience is how afflicted your partner is, how accountable they are, how their behavior impacts you, and how willing you are to tolerate non typical patterns of behavior. In my case, the first was extreme on the scale, while my current husband is mildly affected.