I think it's the rare person that never encounters some form of depression. There's even an entire genre of music dedicated to the problem - the blues. The lucky ones are those who are sentient enough to recognize the creeping onset. Causation, intensity, and curative/restorative mechanisms cover a wide spectrum. If you're lucky enough to know what's happening, but don't think you're making enough progress on your own, by all means look for a professional of some sort, and keep looking until you find one that actually brings you some benefit.

My favorite cure for the blues is to (1) be sure that I force myself to get out and socialize, even if it only means going to my local for a pint and some chat; and 2) start to learn a new song or tune. The focus and concentration required to learn a new tune is just a way to get the mind off feeling sorry for yourself. Plus, there are lots of little "accomplishments" in the process, like mastering something as small as a phrase or pattern in the tune. And then there's always the lyrics.

As everybody already knows from this thread, "everybody gets the blues" sometime.