He did tell me the other day that he thinking he is receiving the wrong type of therapy - that he has been going for 12-15 years will little to no improvement.
Yeah, I'd tend to agree--If therapy is going to be effective, some results ought to be showing up in weeks or months, not years or decades(!) If that kind of comment came up again, I might have taken the chance to ask what he wanted to change, and whether he had mentioned this to his therapist... .but as you said, bringing this up is a bad idea. Heck, even agreeing with what he says is flirting with disaster
On the personal side, we do live in separate houses. We spend a majority of the time together at his house and I do spend the night. We have never had a problem with our sex life - that is one area that does work well. We started eating dinners together because it seemed silly not to - we both had to eat and if we were cooking, it was just as easy to cook for both of us.
I'm glad to hear that one thing is good, at least
That bit about "it seems silly not to eat dinners together" sounds like a bit of justification or rationalization on your part... .I suspect you are discounting arguments/fights at these times, and/or his games around refusing to eat or sleep, and blaming/attacking you for it.
Does the baggage you pick up by sharing means with him justify the "easy" or "time saving" aspect of it?
If you truly enjoy eating with him, and cooking with him/for him, by all means, keep on. But I suspect you don't, or at least don't much of the time.
I am trying to detach a bit from our personal life - I have been pulling back this week - spending time away from his house. He has been telling me that it is making him more and more depressed.
Obviously you have good reasons to do so; Heck, I think I've encouraged you to do this. That said, of course he will notice, and of course he will see it as rejection/abandonment, and of course he will lash out or act out in response. Sigh.
Continue doing it as long as it seems right, and works to protect you. And be aware of the consequences.
All I know is that I am exhausted and generally feeling really bad about myself.
I cringe when I see him name on my phone - when he calls or texts
You know how this is making you feel. Honor your feelings. Believe them. Act on them.
Take time away from him. I'd suggest that for some of it, you completely mute/block him on your phone, so you don't even have to cringe when his name comes up on your phone, saving that for later. Or turn your phone off, if that works for you, and there isn't anybody (besides him!) who you want to hear from.
It is very lonely being in a relationship with someone with BPD. I don't share any of this with anyone outside this forum. I have isolated myself from my friends over the years. It was easier than trying to explain any of this.
Please try to change the isolation part. Reach out to some of those friends, or any family members you like/trust that you've backed away from.
But go light on talking about your r/s and especially BPD with them. People who have some understanding of mental illness, and living with mental illness are few and far between, and as you said, trying to explain can be really hard. (Unless you find a friend who does understand already!)
Instead, have "normal" friends to spend a bit of time being "normal" with them and leaving the insanity behind. Go to a movie, go out dancing, go fishing, work out, go shopping, plan redecorating a house. I don't know which will appeal to you or your friends, but if none of those work, pick something else.