From the Mayoclinic
Persistent depressive disorder, also called dysthymia (dis-THIE-me-uh), is a continuous long-term (chronic) form of depression. You may lose interest in normal daily activities, feel hopeless, lack productivity, and have low self-esteem and an overall feeling of inadequacy. These feelings last for years and may significantly interfere with your relationships, school, work and daily activities.
If you have persistent depressive disorder, you may find it hard to be upbeat even on happy occasions — you may be described as having a gloomy personality, constantly complaining or incapable of having fun. Though persistent depressive disorder is not as severe as major depression, your current depressed mood may be mild, moderate or severe.
Because of the chronic nature of persistent depressive disorder, coping with depression symptoms can be challenging, but a combination of talk therapy (psychotherapy) and medication can be effective in treating this condition.