> I'm also trying to be supportive and help, but I'm often at a loss over how to help. A lot of the time I listen and nod along with "yeah that sucks." But it feels like all I can actually do is try and find the optimistic side or solutions to problems, and most of the time I get shut down as they can only see the negatives. I can't blame them, but what else can I do?
(Caveat: I have experience with people who have other conditions, but not with someone who has BPD.)
Look at the other messages here: What do people need? To be seen. To not carry the burden by ourselves. That is what your significant other needs. See them, be interested in them, be curious about them - just for their sake, as they are, as if they were valuable and lovable and important as they are. Give them space to talk, without any resistance at all from you; wrap your arms around them, physically or emotionally, and let them be and talk.
Leave yourself and your ideas out of it, way out of it. As a crass comparison, it's a bit like looking at a painting or reading a book that you love - you dive into it, you leave yourself behind, you want to know more about the author, the characters, what they mean, what they are thinking ... they have nothing to do with you.
Problem solving is a much different mode - those are your ideas. That has always been my instinct and I've had to learn to leave that behind. Unless someone makes clear they want it, just listen. Your S.O. doesn't need to be right or wrong - it doesn't matter. They are just very, very lonely with their condition.
Imagine you have a problem that is not only awful, overwhelming and terrifying, but others condemn you for it (even though there's little you can do), ostracize you for it. Even your loved ones don't want to know the truth, the reality of it - they can't handle it, amplifying your isolation and pain; they pull away. Don't pull away.