I say depression also. I think depression ften stems from loneliness and then add in cod weather and grey skies and it is a recipe for the blahs. Blahs that can cause what I call brain fog. I think, and this is just me, not some sort of scientific thing I studied. That it can be a gift when times suck that the brain kind of numbs us. Our memory decreases and days blend together. But, it is a good thing during tragedy, things like mourning a bad loss. But, when it is happening when nothing is really wrong, not obviously wrong, no big traumatic event, just not very engaged in life, it actually can makes things worse. It gets harder to climb out of the hole. The depression hole.
Maybe research about the lights that help people with Seasonal Affective Disorder. Plan a trip or something that you can look forward too. Even better if it is to a warm sunny place. Really warm, bikini weather. I have friends who go to tge panhandle of FL in the winter and that is not warm enough in my opinion. Sure warmer than 20 degrees in MI, but they still have their body covered in clothing and the sun does not always shine. South FL, the carribean, MX, sounds better to me. If you always feel cold, kind of need to curl up under blankets and still are cold, turn up the heat. Consider taking some extra vitamin D and iron. I recommend you get those both tested, a blood test actually. You can get it in a typical multivitamin, but it is possible you are very low in both and need large doses. I just never recommend high doses without getting a test. I run very low in both, and getting my numbers into normal range makes me feel physically and mentally stronger.
Really make an effort to make a couple new friends. Ask people over or plan a meal or day out.