What they long for in a relationship has more to do with what they missed in childhood than what a normal adult expects in a romantic relationship.
There are four types of Borderline Personality Disorder each with its own symptoms. Understanding these BPD subtypes will help you better understand the mental disorder.
Temper tantrums are common for people with BPD and managing them is an important part of building a sense of self-control and stability.
If your loved one is recovering from complex childhood wounds or Complex PTSD, they probably experience reality differently than you do. It is helpful to gain some insights into how their world operates, thus making some sense of their behaviors.