As the name implies, a personality disorder is a mental illness that affects your personality. Your personality is everything that makes you uniquely you. It includes what you believe and how you think, feel, and act. A personality disorder is a deep-rooted dysfunctional behavioral pattern that becomes part of your personality and that causes you intense distress, chronic relational difficulties, deviates strongly from societal norms, or otherwise makes it difficult for you to live a normal life. Personality disorders sometimes respond to therapy but less so to medication. The most common personality disorders are narcissistic, obsessive-compulsive, and borderline.(also called Emotionally Unstable Personality Disorder).
The video below provides an introduction to BPD/EUPD. The prime characteristic is a long-term pattern of unstable relationships - particularly when it comes to romantic relationships. Other symptoms include a poorly-developed concept of self, poor communications skills, projecting their own motives and emotions onto others, and "splitting." Splitting refers to a tendency (usually unconscious) to view most or all aspects of one's life in a false dichotomy of either extremely good or extremely bad. An individual prone to splitting is typically quick to either idealizing or devaluing people, things, and situations. BPD/EUPD people habitually jump to wrong conclusions about other people without attempting to resolve misunderstandings by talking things out. According to psychologist Jordan Peterson, people with BPD/EUPD often recognize their dysfunctional behavior. But they lack the ability to change it.
For other educational videos on mental health, see the "Psych2Go" website. If you are specifically interested in BPD/EUPD, check out the excellent website of National Education Alliance for Borderline Personality Disorder