Hi arugula367 and welcome

It
is difficult to explain to others about staying in a relationship with a partner who is coping with mental illness. Yet many members here stay, for many different and personal reasons. We understand that your choice to stay is your choice -- so the questions pivots from
if to stay, to
how to stay healthily.
Does your partner have any "official" diagnoses? It's interesting that he's open to doing a BPD workbook -- did he suggest that?
There are definitely choices you can make to improve your own experience in the relationship. It can take a pretty big mindset shift, but like you're finding out:
I don’t feel like I can give him the emotional support that he needs without abandoning myself completely. But I am trying, and I feel like I can’t stop.
If what it took to stay in a BPD relationship was unceasing giving of yourself -- if that really worked -- then this group wouldn't be here. I think you're finding the wall that many partners find, which is that the "intuitive" move of "if I was just more supportive, things would work out, he'd be better" doesn't improve the relationship.
One of the unintuitive aspects of staying in a BPD relationship is that the partner without BPD has to work on themselves a lot, to have a strong sense self, to tolerate the discomfort of accepting that they cannot change how the pwBPD feels. We have to radically accept that the pwBPD is allowed to feel how they feel, and we can try to find moments to
connect safely via
emotional validation, and we must choose to
protect ourselves from toxicity via
genuine boundaries.
...
This is standing out to me as an area where you can try some immediate changes:
He primarily directs his hatred outwardly into things about the world he dislikes (most things: social media, capitalism, the work force, party culture, hookup culture, etc) and towards himself. I feel like he has made me into more of a pessimist than I was before, that his ideas about the world and the hopelessness of the human condition have infiltrated my consciousness. I’ve tried to show him my world, to show him beauty and meaning, but he is rarely receptive.
Can you talk me through what this looks like in real life? For example, does he text you those feelings? Start talking to you when the two of you are at home? Call you on the phone to tell you?
How long does he continue directing his hatred outwards -- one minute? Ten minutes? Two hours? Other?
In the past, what have you done when he starts in on his hatred? Do you listen? Engage? Disengage? Something else?
I'm confident that by working on that specific issue in the group here, we can brainstorm a way to change that aspect of your relationship to be better and more tolerable for you.
Fill us in on how things have been going;
kells76