Borderline personality disorder (BPD). Its a very hard condition to live with and very stigmatized, even within mental healthcare (some therapists refuse to see patients with BPD). I have a friend with the condition and I love her very very dearly; she works her ass off to manage the condition, which I imagine is a little easier now that she knows she has the condition and can understand a bit more what's going on and what might be productive in improving things.
The reason its relevant is that a large component of BPD is relational volatility. And additionally, an unstable or fluctuating sense of self, persistent feelings of emptiness, relational paranoia, intense emotions and difficulty regulating them resulting in emotional volatility, impulsivity, and higher risk of suicidality and other impulsive high risk coping strategies (dangerous driving, risky sex, self harm).
Though in the perspective of my friend there are also other large components of the condition that are less often engaged with or discussed because they aren't what affect other people, which she feels has been much of the focus in how people try to adress BPD, rather than the patient's experience of distress, not all of which is relational. For example psychosis is very common in people with BPD but very rarely discussed. She has explained BPD as being, in some ways, like a cocktail mixture of other personality disorders.
It's also my understanding though, that when the patient is seeking to adress the condition (which is not always easy with personality disorders), BPD is very responsive to treatment, which is really cool :)
https://www.nimh.nih.gov/health/publications/borderline-personality-disorder
https://pmc.ncbi.nlm.nih.gov/articles/PMC9005124/
Please take all of this with several grains of salt, this is intended to be a rough overview because my love for my friend gives me an interest in talking about the condition compassionately, but I'm not a researcher or healthcare professional. I just care about my friend a lot, and much of the therapy I've done (DBT, dialectical behavioral therapy) was developed in part as a treatment for BPD