I used to hate the summers here in Phoenix over the decades i have learned to appreciate the weather here as a whole instead of focusing on July and August. Clearly I can't empathize with your situation, heck I miss the sun after 1 or 2 rainy days although there were many years I " thought" I wanted to shovel snow. Dealing with no sun back there for as long as you do sounds like a big challenge. There's no magic bullet for the blues IMHO, starting with one small step has always helped me to take another. Load that tool belt with all the good tools you need to put a smile on... grow lights, hobbies, new experiences and relationships are a must too, we are hard wired to be social for sure. My wife and I have been through a lot, and for the bigger stuff we chose intensive courses on communications and leadership. Some of these done together some individually all of which helped both of us keep moving forward and deal with life's challenges. Exercise has always been huge for me, I've been mountain biking for 10 years and it beats stress, boredom and the blues every time. I sure hope things improve for you, I get that way too even without the gloomy weather and it is surely no fun.
Hang in there