Hi placebicstar,
My partner w/ BPD has it in their mind that nobody cares and nobody acts like they care. While I try to validate the feelings and see things from their point of view they're convinced nobody cares. A lot of it comes from me viewing interactions they have with others and seeing where the feelings come from (like a friend of ours would give them the cold shoulder, or ignore them when they say they have a bad day, or ignore both of us when we want to do things with our friends) and I can see a lot of what makes them have episodes (as we call them?)
we all are being disappointed all the time to a degree. Life sucks to a degree (and it is great too but one who is depressed does not want to be reminded). You can't fix this. You can't make it better. Your partner can't fix it. Some of it just is the way it is and needs to be dealt with. Validation of your partners emotions is the one thing you can do. The other is to avoid owning the "problem" and letting your partner deal with the hard realities of life.
We've both been working on their triggers and I try to view and identify them where I can. I think we've made baby steps of progress.

Allowing your partner to grow beyond them. It takes many baby steps to grow up from a teenager entitlement stance.
Recently something happened and it convinced them to isolate themselves from everyone, telling me to go away and not talking to me for a few days. I understood to give them space, as they are their own person, but I am concerned about their well-being. But there's also not a lot I can do as we are a long-distance couple.
Finding a way to ask and give space without triggering abandonment on either side is a valuable achievement.
Is there any way at all I can help them further?
Help yourself! Let your partner struggle as long as your partner does not fall badly. The stronger you are the better position you are to help at times it truly matters.