I can sympathize with the OP and the Seattle dweller - I was in Seattle one February when the sun came up at 10 am and went down at 4pm. It was cold and drizzly and snowy and overcast the whole time. I had flown up from for a meeting Hawaii (where I'd lived for 20 years), and I wanted to poke sharp objects into my eyes!
Great advice from everyone. I especially appreciate the earlier post on Aspergers. You just cleared up a mystery for me with my little sister who has that. She loves living in South Dakota, and loves staying indoors during the winter. Couldn't hack California.... very helpful, thanks.
Everyone has different causes for their depression - some of it is emotional, but most is just chemical. You will find this weird, but I discovered after some testing that I'm allergic to cheese. Not soft cheeses, but the hard ones. Apparently there's an enzyme that drops your seratonin levels. Somehow milk+alcohol has the same effect on me. So my solution is simple - avoid beer and pizza!!
A couple of thoughts to add to all the very good advice about Vitamin D and lights -
first - here's the link for the Happy Light - which I bought even for those 35 cloudy days in Phoenix
Light Therapy Lamps for Seasonal Affective Disorder (SAD), Sleep Disorders & Skincare
Someone mentioned Oil - one of the best I've seen is
Udos 3-6-9 Oil. You may be able to buy it online if your store doesn't carry it.
I get packets of vitamins from
jigsawhealth.com - it's an interesting site pulled together by the guy that invented ACT contact software. He struggled with a lot of health issues including depression, and he has pulled a lot of medical data together in one place.
I make sure i take my 5HTP every day. I stay away from foods that make me feel crappy, and i make sure I get a lot of sun and exercise. Your diet has a LOT to do with how you feel. Apples have great enzymes and wake you up better than coffee or tea in the morning. Whole wheat and veggies are great (unless you have a wheat allergy) -- DM me if you want some great recipes. I posted one in the South Dakota "whatcha eatin'?" thread.
I would get checked by a non-traditional doctor who is willing to look at food allergies for the root of your depression. If you are near Santa Rosa, California,
Dr. John McDougall is an internist and allergy specialist who figured out my problem for me. His website is also packed with great information, non-allergan food you can order, and a wealth of data.
One more thing - make sure you are replacing electrolytes if you are in extremely hot or cold weather. I got rid of all the table salt in the house and only use
Celtic Sea Salt. It is lower in sodium, but very high in electrolytes and trace minerals. We are always told to drink a lot of water, but don't think about the trace mineral loss that's lost when you have to

- can i say that here? LOL
Anyway - there have been really hot days here in Phoenix where I suddenly just feel awful. Lose my motivation, don't want to do anything. First thing I'll do is put some celtic sea salt on my tongue. I've seen a shift in mood in about 5 minutes from doing that. You can also make your own 'gatorade' by mixing a teaspoon of the salt into your bottle of water. I've done that when i'm going to be out and about in 115 degree weather.
One more thing - in response to some of the posts about managing your emotions. There is a wonderful book called
"You Can Heal Your Life" by Louise Hay. That has also been a wonderful source of inspiration and information for me.
BTW - full disclosure - I specialize in Stress, Overwhelm, Work Life Balance, and Resilience. That's why I have a lot of notes on this stuff. Hope you can find your answers somewhere in the midst of all this data. (Never ask a speaker a question... we'll give you a freaking seminar!! And you don't want to sit next to us on an airplane!!

)
Good luck!