A 48-year-old female teacher presents reporting she has 'always been a sad person.' She describes a depressed mood present more days than not for approximately 6 years. During this time, she has experienced poor appetite, low energy, difficulty making decisions, and low self-esteem. She has continued working and maintains basic social connections but describes life as 'going through the motions.' She denies any period during these 6 years where she felt well for longer than a few weeks. She has never had suicidal ideation, psychomotor changes, or significant weight loss. She has never experienced a manic or hypomanic episode. Her PHQ-9 score is 11. Which diagnosis most accurately captures this clinical picture?