There's such thing as dysthymia (persistent depressive disorder). About 1.5% of people have it. Treatment is supposedly as effective as treatment for depression (it's the same treatment). You have to feel like that for at least 2 years to get a diagnosis. Since it's not severely handicapping people suffering from it, it may go undiagnosed for decades and even after diagnosis people might choose to forgo treatment because of its hit and miss nature and side effects.