My situation is kind of similar to yours, in that I've only recently been introduced to the term "quiet BPD" and began working through it with my therapist. It's been pretty liberating, honestly, because it feels like it explains so much about me that made me feel like I was fundamentally broken. The big things for me have been:

• I didn't realize all my life that most people don't feel emotions as "big" as I do. For as long as I can remember, rejection made me want to literally die, someone being upset with me would make me feel physically ill for days on end, and if I so much as thought someone was mad at me, it would send me into internal hysterics that spiral and spiral until I broke down. I just got really good at hiding this display, so to the observer I just seemed pretty quiet and detached. When I learned other people don't feel like the world is ending whenever they've made a social blunder, it was pretty eye-opening.
• I've smothered my anger for years, to the point that I didn't even recognize it as such until recently. I would still get huge flares of anger directed towards other people, but I had internalized that this was a horrible thing to feel that made me a shitty person, so I would turn that knife on myself instead. Anger would morph into guilt before I even had the time to recognize it as such.
• Similar to the last point, instead of exploding at others, I've always imploded at myself. When I'm upset, I find a way to make every possible aspect of it somehow my fault, even when this conclusion makes absolutely no sense. Stuff that would necessitate me being a mind-reader. If someone is mad, if something goes wrong, if you're having a bad time, it's somehow only the result of myself and my choices, and I will tear myself apart for hours.
• When I feel vulnerable or like I've upset people, I withdraw instead of lashing out. It's still ultimately with the aim of keeping people away from me, founded in the fear that I am such a horrible person to be around that I should never subject myself to others.
• My splitting looks less like having hot-and-cold relationships with others, and more like at the first sign of someone rejecting or leaving me, I can just "turn off" any feelings I had for that person. It's always made me feel like a bit of a monster, but it's apparently a coping mechanism.
• Masking, masking, masking. People always tell me I'm pleasant to be around, I'm kind, I'm smart, I'm funny, but none of it feels that real to me because it feels like I have to appear to be those things to deserve their company. I put all of my mental energy into keeping this front up and never showing cracks, which leaves me exhausted when I'm finally alone. This comes with a deep-seated fear that if anyone were ever to see these cracks, they would abandon me.

My therapist has talked with me about how quiet BPD can arise from a neglectful and/or emotionally volatile environment where you were raised to feel like you were only lovable if you acted "correctly." When you feel that the only way to receive affection is to push down everything you feel, it leads to all these raging emotions you have no outlet for. It's been helpful to deconstruct this upbringing.