I feel for ya and it can be a tough battle. In my opinion there are many factors, and it's like californialaw said, location alone can do a lot. If you don't live somewhere south where there is lots of sunlight already, I would recommend getting into a tanning bed at least twice a week. Lay in the ones that last the longest, not the most powerful.
Also getting some natural rays does the body good as well. Even if you are already dark skin, something about those UV rays or the toxins you sweat. THe highest suicidal rate is in alaska for a reason....
I also find diet to play a large role. One milk bearing cows and constantly given steroids so that they produce more milk. This in turn shows up in our milk, whey, cheese, and beef. This then gets into our bodies and fucks everything up. We also have fruits and vegetables sprayed with pesticides. Some pesticides are actually estrogen that make it so the bugs can't mate. You also have modern grains, such as bread. If you think about seeds any seed, it is a self preserved life form that is waiting for ideal conditions in order to bloom.
Generally dark/warm/wet ( underground/ spring or summer/ after rain or watering ) condition is when a seed will sprout. It then starts to pull vitamins and minerals from the soil and grows. This is partly what makes modern bread so horrible, because it's just ground up, and while it obviously isn't going to sprout in our stomachs some of those chemical reactions still occur. This is also why most breads and cereals are fortified, because they steal so many nutrients from the body. AND the reason why ezekiel bread is so much healthier is that they partly sprout the grains before the bread making process, sprouting turns off a lot of the vitamin and mineral stealing properties of grains.
So what do you do about food. Well first buy organic meats, preferably straight from a farm. If this isn't possible, then try and rotate your protein sources as much as possible so that you limit your exposure. Buy a bulk protein, if you can get a guarantee the cows aren't treated with hormones that would be even better, they are out there. Rotate bison, pork, beef, chicken, turkey, ostrich, different types of fish, shrimp, scallops ect. Avoid the fruits and vegetables that are the worse pesticide offenders.
I spray all my produce with a apple cider vinegar and then hydrogen peroxide. It may sound silly, but it takes the waxes off very easily. Last look more into sprouting your grains. I recommend the book nourishing traditions. It was actually very helpful with giving me ideas for weight gain as well. Like a whole carrot cake before bed =)
Another factor you may want to look into is leaky gut syndrome. An unknown fact is about 90% of serotonin is produced in the intestinal tract. I believe l-glutamine and zinc to be the most helpful supplements to repair a leaky gut. Also make sure you chew your food well. Don't take 2 bites and swallow like I did for years!
If you don't mind shelling out 4000$ a strongly recommend an ion panel. It's an extensive blood test that finds any food intolerances, amino acid vitamin or mineral deficiencies you may have and much more. If you don't then you could look into something like the blood type diet as they kind of go hand in hand.
Last make sure you work out for a minimum for 5 hours a week. We were designed to work and work hard. Yet we are becoming lazier and lazier, staring at computer or tv screens. Working out regularly causes many positive hormone changes. I personally found I need 5 hours of hard gym time a week in order to stay mentally positive. Thing is, the more I lift, the more i want to lift.
Pay close attention to your food. It sounds like a lot of work, but depending on the severity of your depression it can definitely be worth it. I been there, and determined to stay away from a pill I read and learned as much as I could. Nothing beats nutritious food, sun shine, and heavy iron. Some ideas may seem out there, but there is plenty of support for all the claims if you just google them. Good luck, hope you start feeling better soon!