A 34-year-old female reports feeling "down most of the time" for the past 3 years. She describes low energy, poor concentration, low self-esteem, and difficulty making decisions. She has never had a period of feeling "normal" lasting longer than a few weeks during this time — she has not been symptom-free for more than 2 months at a time. She denies discrete episodes of markedly worsened mood, weight changes, psychomotor changes, or suicidal ideation. PHQ-9 score is 11. What is the MOST likely diagnosis?