Sassy, I’d like to join AbuNassif in saying welcome to the family!

I’m so sorry for what you’re going through. It’s so painful to be rejected by those we love. While my H hasn’t left, I, too have been accused of not being supportive, etc. And I suspect he’s been listening to people who don’t know the real story saying things that just feed the unhealthy dynamic.
Therapy can be a great thing. But BPD is difficult and if you get someone who doesn’t “get it”...
Unfortunately, if your H doesn’t want to work on it, there’s not much you can do to change his mind. But, there are tools you can use and things you can do that may either lay the path for a reconciliation (not a guarantee — if he’s set on leaving, there’s not much you can do) or make you stronger for whatever lies ahead.
So, when you meet up or talk on the phone, how does that usually go?