The Borderline personality doesn't just dissociate from difficult emotions, they separate themselves from light, pleasurable ones as well.
Central to BPD traits is self-sabotage. They are their own worst enemy. Self-loathing is tantamount to why it's impossible for someone with BPD features to choose a healthy, grounded, nourishing relationship experience. It simply doesn't match their inner sense of themselves, and what they feel they deserve.
Moments of joy are always short-lived due to repeated childhood programming that conditioned them to expect disaster soon after they felt stable or good. Anxiety is experienced in preparation of "BAD" happening.
When the Borderline can orchestrate their own setbacks, they get to feel some semblance of control and prophesy fulfillment satisfaction~ rather than being caught by surprise by any imagined impending disaster. Shock and surprise were feelings states they had to accommodate, and even learn to love as young children, in order to survive those episodes. This issue is central to their addiction to intensity, chaos, instability and drama.
The Borderline's "normal" state of being, that which they're inured and accustomed to, is struggle and strife. This is all they've come to believe they deserve.
When there's an absence of struggle and painful feelings, it doesn't feel natural or normal to them. Their compulsion to engage in self-defeating behaviors in order to catalyze pain for themselves, causes acting-out behaviors (like dating, when they have no capacity to give OR welcome real love).
When anything GOOD comes their way, like financial gains, healthy romantic attachments, business successes, etc., they find ways to undermine those gains, due to feeling unworthy or undeserving of genuine and lasting happiness. This is why they'll break off a connection with you, when your relationship seems to be going well.
The Borderline's need to self-sabotage when life seems a little "too good" is what keeps them circling the proverbial drain, and never gaining enough self-worth to climb out of the sink. These patterns of behavior are so deeply entrenched, few will surrender them in order to become emotionally healthy and well.
Love, prosperity, joy and contentment then, remain elusive to these people, regardless of how bright or skilled or talented they might be.