Yep, I've been there, sometimes for extended periods. As you noted, nihilistic moods respond to physiology - food, exercise, carbohydrate. I'd like to add that they also respond to dietary deficiencies (in my case, thiamine, though I'm biochemically atypical); and that taking a multivitamin regularly tends to be one of the first things to go with self neglect. So if you find yourself there, and haven't been taking multivitamins lately - go get some, and make sure to take one every day. It might have a surprisingly disproportionate effect.