My teen lacks a lot on sense of identity. Does this mean when they change all the time, almost cycling through personality changes she may have Borderline?

A lack of a stable sense of identity is indeed one symptom of Borderline Personality Disorder. However, an unstable or absent sense of identity is also quite common among teenagers, even those who will not later get a diagnosis of Borderline Personality Disorder. in order to be diagnosed with BPD the presence of at least five symptoms out of a list of nine needs to be established by a clinician or therapist. These nine criteria are the following: 1. Frantic efforts to avoid real or imagined abandonment; 2. A pattern of unstable and intense interpersonal relationships characterized by extremes between idealization and devaluation; 3. Markedly or persistently unstable self-image or sense of self; 4. Impulsive behaviour in at least two areas that are potentially self-damaging (e.g., spending, sex, substance abuse, reckless driving, binge eating); 5. Recurrent suicidal behaviour, gestures, or threats, or self-harming behaviour; 6. Emotional instability in reaction to day-to-day events (e.g., intense episodic sadness, irritability, or anxiety usually lasting a few hours and only rarely more than a few days); 7. Chronic feelings of emptiness; 8. Inappropriate, intense anger or difficulty controlling anger; 9. Transient, stress-related paranoid ideation or severe dissociative symptoms. If you suspect that your daughter may suffer from BPD, it would perhaps be a good idea to have her assessed.