hi stexia,
whats clear is that youre in a difficult place, without a lot of resources, and i can imagine its a pretty overwhelming place to be.
the good news is that youve found support. things can start to get better.
a strong support system is probably one of the most vital things to have, when it comes to loving someone with BPD. the emotional roller coaster can be confusing, hurtful, and it can really mess with your nervous system.
since our loved ones inherently lack good coping skills and relationships skills, it can be hard to sort of steer the ship with that going on.
as kells mentioned, there are a lot of moving parts here, that didnt start over night, and wont be solved over night. its important, in this process, to have a realistic understanding of what progress can and does entail, and realistic expectations when it comes to it.
the good news there, is that it seems like a good process is in order. hes getting some help, it sounds like the two of you are (or were) together. thats not a guarantee of course, but its promising.
that he is struggling with the process, or that he threatens to quit, on some level is to be expected. as big as the difficulties in your marriage are, he has a lot going on an individual level, and those things wont get better over night. we have a thread here for parents with a child with bpd; it talks a lot about what to expect when your loved one is in therapy, and while its written for parents, most of it will apply:
https://bpdfamily.com/content/support-child-therapyHe is in counseling but everytime he splits he wants to quit.
there could be an opportunity here.
what happens when he does this? lets say he comes home, and he starts telling you how much he wants to quit counseling. how do you respond? how does it play out?