Oh now. This sounds very serious with the disassociation! I am glad you have some professional help/eyes on this!
I know what it feels like to think you have to hide/guard your own emotions because their emotions are so dysregulated. Expressing them here can be a good outlet. Just keep it up!

You are inspiring me to do the same with my backlog of resentments over exactly this - not being able to fully express my emotions.
I find that with my own thoughts it helps if I put positive thoughts into my head instead of worries when I can see/hear too much negativity in my head. I've had a few tricks with this since I was kid, but I am finding that they are still useful! I keep a list of little mantras of words that make me feel good that I can repeat to help bring my thoughts back into balance. I like this one when to remind me how I want to be: "Be kind when possible. It is always possible." (-Dalai Lama) It reminds me to simply act with kindness as my guiding principle no matter what storms this BPD stuff throws at me.
Oh, and you have a great attitude about this already - that you don't want to see him suffer! Having compassion will definitely help you will get through this. It is an ongoing process, with lots of ups and downs, but don't let knowing that overwhelm you - the future is not here. Take it day to day and be in the present with it.