| It could be something that's a bit transient. We're all prone to periods where motivation wanes and I know this time of year can be tough on me at times as the daylight hours dwindle.
Still, I will say one of the best things I've done the past couple of years which has helped my wintertime mood immensely is getting and staying active as much as possible. That's had a very real effect.
Perhaps what you've been doing for activity has hit a sort of routine or staleness. Try changing it up maybe. Walk different routes or at different times and see the differences. Good exercise not only engages the body, but engages the mind and I think sometimes it's good to shake up the routine so the mind has something new to mull over. | 
