Perhaps you're affected by Seasonal Affective Disorder? Many medical supply companies rent light boxes for a month at a time.... you could see if that helps before contemplating purchase. I used one when I lived in Seattle for a couple of years and thought it was a lot of mumbo jumbo so I was really, really suprised that it helped with mood so much.
Also, and this might not be true for you, but I found that the more time I spent thinking about how miserable I was the harder it was to break out of the cycle. Exercise helped quite a bit, even when I didn't feel like it I'd force myself so at least if I was miserable blob I was doing something to keep me from becoming a flabby miserable blob. Exercise hard and long enough at something aerobic and the brain churns out the feel-good endorphins.
Another thing that helped sometimes was getting out of my self-centered mindset by volunteering in my community. There are a lot of things one can volunteer to do that don't require a lot of interaction with people. United Way offices, Red Cross & the like are often able to suggest organizations that could use some help with things like stuffing envelopes for a charity office or maintaining the snack area at blood donation centers, for example.
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When two elephants fight, the grass and trees suffer. -- King Sunny Ade