A 28-year-old woman is referred to psychiatry by her general practitioner after presenting with recurrent self-harm and unstable relationships. She reports intense fear of abandonment and has had five romantic relationships in the past 3 years, each ending after intense conflict. She describes her mood as "like a roller coaster"—shifting from euphoria to despair within hours. During a recent argument with her boyfriend, she threatened suicide and engaged in cutting. She denies suicidal intent currently but admits to chronic feelings of emptiness. She has a history of impulsive spending, binge eating, and occasional substance use. Her family history is significant for depression in her mother. Mental status examination shows appropriate affect but marked emotional lability. What is the most likely diagnosis?