Are we talking real 'clinical depression' or just the occasional 'blahs'? I was probably somewhere on the spectrum between the two and was pretty sure it was being caused by new management at my job that seemed to insult and belittle me as often as they could (I was the oldest person there, so it might have been age-related). I finally just retired a little earlier than I had planned to do and life has been a whole lot better. I've gotten a few phone calls from the old job but just ignore them all.