Am I depressed or just sad frequently?

For at least 2 years now I've gotten periodically really sad. I never stop living "normally" however, i continue my activities. I eat and sleep relatively the same besides some cases of insomnia and constant nightmares. It lasts usually just a few days, but can be a week (rare). Sometimes it's triggered by disappointment from my incompetency in life, sometimes there doesn't seem to be a reason. Sometimes it's almost like a relapse from being overly social with friends and subsequent guilt over not being "productive." My thoughts during these periods aren't characterised by hopelessness, but pointlessness. If we're all going to die, there's no point in living. But then again, if we never died, there would still be no point. Sometimes I think about death but I wouldn't say I'm ever suicidal. I also cut myself, but other than that I wouldn't say I have any extreme symptoms. Also, lately my sadness has cycled so rapidly within the day I'm sad once or twice per day and the rest of the time pretty happy. Thoughts?