Hi CarolAnne82 and welcome back

Breakup/makeup cycles can be common here; you're not alone. And everything you've described about your relationship -- the neediness, aggression, anger-to-tears, suicide threats, demands for you to be there, etc -- will ring true to members, too. I also understand that there are likely many positives about your SO and your relationship (you mention that your SO can be a caring person), and those make it challenging to decide on a path forward; it's not all bad all the time.
It certainly sounds untenable to keep giving so much emotionally and not to receive support in turn. While BPD relationships will likely always be unevenly balanced (as "emotional special needs" relationships, partners will probably not achieve close to a 50/50 split of support/care), a 0/100 split isn't something anyone can sustain.
I want to do my best for my partner and know I can't do that without caring for myself, but I feel unable to care for myself because it regularly starts a conflict.
Can you describe that some more? For example, does this come up in conversation? If so, how does that typically go? I.e., writing it out like a script:
You: I think I'm going to ________ (take a long bath, go for a walk, start therapy, call my friend) on Friday.
Her: You never think about what I need -- it's always about you. Why can't you just not be selfish for once?
You: But I always support you. Remember last night when you were crying for 5 hours? I was there the whole time!
Her: But you didn't really mean it, I could tell. I need you on Fridays and you're just going to abandon me?
You: OK fine, I can be with you, it's fine.
Understanding this conflict can help us work with you to find a path forward. It might not be easy or comfortable, but I'm confident there are different ways to approach it.
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Previously I had broken up with her because she is aggressive, threatening, and will not respect my boundaries. We recently had a disagreement about our relationship timeline (she wants to get married ASAP and I do not). She is unable to accept my answer
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I broke up with her a month ago but we talked and got back together but I still feel anxious about the relationship.
How did that conversation go? Did you two address behaviors, or relationship timeline?