I'm going to throw out a little bit of whackiness.

How isolated are you from nature? I have been a terrible sleeper most of my life but two things have made a huge difference. One is diet, but I know you are spot on for that. The other is being in touch with the earth. In the summer, despite a lack of darkness in my northern location, I sleep MUCH better than in the winter. I attribute this to my ability to be outside while barefoot. If you can, walks at dawn and at dusk, barefoot, can really help sync your circadian rhythms to those of the earth. If your rhythms are cycling properly, sleep comes more naturally. Yes, it's a little hokey, but it's free, safe and easy enough to try.

Living in cities, surrounded by concrete, in high-rise buildings, always wearing shoes (and sometimes super high heeled ones!) only isolates us even more from nature. Breathing fresh air and exposure to sunlight (both known health promoters) are only part of the equation. Even astronauts are intentionally exposed to the same vibration frequencies as those of the earth to ward off sleep problems, irritability and other maladies.

I do agree that getting to the root cause of your stress/depression will have the biggest impact on your success, though.