Just chipping in as I noticed the mention of SAD on this SE thread.
I noticed a few years back during the darker months I was lacking energy and often had a pretty lousy mood. I'd gotten past teenage moodswings and this felt much more like a mild depression which seemed to recur every winter.
Not that it will work for everyone, but I tried taking occasional vitamin D supplements (available on amazon - I chose to use the vitabiotics ones but there's plenty available), and they had a profoundly positive effect on both my mindset and physical energy level. Please, if you're feeling a bit down and haven't tried these, or sun-lamp, give it a go. It doesn't stop the pessimistic thoughts of - 'bloody heck will this rain never end?!' But at least you might smile about it.
Fingers crossed for some high pressure and clear frosty winter days soon ey.