Hi saltysea and welcome to the group. This is definitely a place that understand that a BPD relationship isn't a "normal" relationship. Just a few questions to get a better picture of your situation...
I'm married. My partner self-diagnosed himself bpd several years ago after many turbulent years into our marriage. The turbulent years were outweighed with the amazing times we had (in my opinion) and our love grew stronger.
How long have the two of you been together?
Were things turbulent right from the start, or did it pick up gradually over time, or...?
His therapist at the time agreed with his diagnoses. We eventually moved overseas and he went a few years without a therapist. During those years he decided that he was cured of BPD... eventually he decided that he didn't have BPD at all. It was around this time that things became significantly more turbulent again. While we grew closer, the intense meltdowns became way more intense. Eventually I convinced him to see a new therapist again. It didn't really work out. After months of seeing her, he would paint a totally different picture to her in his sessions than what was really going on. It was as if he was ashamed at the idea that he could still have BPD.
Now he sees her every few months and they have discussed winding down their sessions.
Did the two of you try any couples therapy at all? And are you in any type of therapy/counseling/support group yourself?
I'm at a complete loss. Almost every day or every other day now he has huge epic meltdowns. Every thing I say seems to make these meltdowns worse. He flings himself around the room and tells me that I don't have empathy. That I don't care. He has begun to call me autistic and tells me he doesn't love me.
The meltdowns feel SO personal. It's really hard to detach myself and comfort him. He begs me to comfort him and I become speechless. When I do attempt to comfort him he says I'm not talking TO him but AT him. And it goes round and round.
Eventually, sometimes after a few hours, a day, sometimes a few days, he will come back around and apologize. But its usually a matter of days before the next big flare up.
I really don't know what to do anymore.
I'd be at the end of my rope, too. This current situation sounds exhausting and unsustainable, for both of you

Are the "meltdowns' what you're talking about when you refer to things being turbulent -- or is there more (i.e., if you feel comfortable talking about it, is there any substance use, self harm, etc)?
The highlighted parts in your post above really stood out to me.
It makes a lot of sense to me that by the time he's past his explosion point, there won't be anything you can
say to "help" or "talk him down". He won't be rationally reachable.
This is something that many people experience, not just pwBPD. I've experienced struggling to process verbal information when I'm in a high emotional stress situation. For a pwBPD, perhaps it's that "normal" experience, but x 100.
And, when he's past his limit and experiences you saying things that don't seem to be impacting his emotions, it kind of makes sense that he would be angry and feel like you aren't helping or aren't being empathetic.
It isn't because you aren't trying -- it might be more because his brain is not in a place to be able to process what you're saying/doing as helpful and loving. But he might not understand that he can't process what you're doing, so it comes across to him as "saltysea is totally unconnected" (even though it's his rational mind that isn't connected at the moment).
All that theorizing to say -- sometimes, trying something totally different can help. Before we can make things better, we can decide to try something new to stop making things worse. Again, not because "it's all your fault for not having the magic tools or words" -- it's more about how pwBPD really process and perceive things so differently, especially when emotionally jacked up, that we need to try new, unintuitive tools and skills for these relationships to work.
...
Do you think you have a sense of when he's about to melt down, but isn't there yet, vs when he's fully in meltdown?
One idea for something different to try could be to make a
Support, Empathy, Truth statement before he's fully in meltdown:
"Babe, I care about how you're doing. It would hurt so much to feel like I don't. I'll be able to support you when I get back from taking a walk in 45 minutes."
This new approach would let him self-soothe during his meltdown, and would communicate it not as something "negotiable", but as you firmly but lovingly sharing what you plan to do.
Could be worth trying finding ways to exit the space when he is melting down, given that there really isn't much you can say that seems to help him. It could also be loving to allow him space and time to build his own regulation and management skills.
You know him the best -- what are your thoughts?