I agree with what others are saying about cross-training at another activity. I know people that switch to training for triathalons, running, martial arts, tennis, etc. for whole parts of the year. I usually hit a mini-wall about the middle-end of October. Even doing radically different types of riding does not help at that point (steep hills, flat-intervals, long-distance, 1-hour time-trial type riding, mountain biking, etc.).
When I hit that wall, or feel it coming on, I tend to start doing lots of gym and aerobic class workouts to try to keep some of the aerobic conditioning that I have built up over the summer. Eventually, I always end up in group Spin classes by the beginning of November. It is often too cold for ME to ride outside then, but they have 4-5 hours of spin class every day during the winter-months at my gym. I use it to prepare my aerobic engine for the outdoor season too.
Take a break. Cross train with something else that you like to do that is radically different than cycling.