Popularly, depression is often dismissed as addiction to self-pity. Depression is more officially classed as a mood disorder. From "mood disorder" I infer a degraded ability to influence one's own moods. (I realize this isn't an answer to your question.)

I'm glad you are still able to function well and that common physical symptoms do not trouble you. Nevertheless, "feeling like crap" over the course of months, or even years in your case, is an indicator of something.

Have you parsed out your feelings and their effects in a list you could discuss with a medical doctor?