I feel like this every year, around the end of August. This year, it has just started happening in the last 2-3 weeks. I credit this to 1) running a couple of days a week to take the "focus" off of always needing to ride and 2) trying to view my fitness in a more holistic way, by considering everything I do, even stretching, as important.

Two years ago, I got sick and dropped some speed. I mostly got it back by the middle of this season, but all of a sudden, my body seems to be telling me it's time to slow down for the season and switch gears. But, the weather has been nice, so I figure another few weeks of riding outdoors. Then the bike comes inside, goes on the trainer and I focus on other sports. As someone else says, there will be a warmer day in January or February, where I usually suit up and get on my flat bar road bike with sturdy tires and feel very happy to be riding again.
It's all a cycle.