Hi nullset, welcome to the Personal Inventory board!
This is a very interesting question.
Some people identify with the concept of a Highly Sensitive Person (see Elaine Aron's work) or being an "empath" (which there is some overlap but not quite the same). In short, it is the idea that some people's nervous systems are more strongly affected by other people and the environment, including emotions.
I am also reminded of the references to Bowen's concept of "differentiation" (referred to here
https://bpdfamily.com/content/codependency-codependent-relationships) where, in unhealthy families, members don't learn (or don't fully learn) that you can feel your own emotions and they are distinct from the emotions of others. You might ask yourself whether, in your family of origin (FOO) you ever felt that, instead of getting to have your own experience, you had to remain "tuned in" to the emotions of a parent to avoid being hurt or (emotionally or physically) abandoned?
Another thing you might want to look at is the idea of insecure attachment. Particularly with the anxious style of attachment, the person is said to "confuse activation with love". The way I understand that concept is, that exciting feeling when you meet a new person, desire (not necessarily its fulfillment)... .longing for someone who seems just a bit unattainable. This would have been similar to the way the anxiously attached person felt with their parents, who likely provided inconsistent empathy/attunement. So the child begins "attachment activation strategy" behaviours, i.e. to ramp up attempts to get the parent's attention/care (unlike the avoidant, who essentially gives up on getting connection) and as an adult, confuses that "ramping up" feeling for love.
So I don't have answers, but I think you're asking yourself the right kind of questions.
Is it possible to feel this joy directly? (a possible underlying question being "Is this joy part of me or is it "caused" by another person?) And should I find healthier ways to feel it or accept that I have to live without it?
Oh, and yes, I do relate to your experience. Would you say that the relationship with a pwBPD... .seemed to offer the promise of returning to you a joy that... .you are vaguely aware on some level that you "lost"?
