The shorter days, the lack of light, I think they affect many people. It's a struggle to get past it and keep moving. I do think keeping moving is the key. Inside, outside, doing something active helps. I, too, would always prefer to ride outside, but I'll take indoors over nothing.
Spinning, for me, is more motivating than riding my trainer at home. You are with other people and that makes me ride harder. A lot of spinning is what you put into it. As an instructor, I try to encourage the class to ride strong and hard, but it's really up to each individual how much they want to put into it. When I am out in the class, riding, not leading, I concentrate on making it as close to a real riding experience as I can. It's easy, on a spin bike, when it starts to get tough, to turn that resistance down, but real hills don't get easier or flatter when you get tired, so a spin climb shouldn't either. It will never be like riding outdoors, but I truly believe it can keep your legs strong throughout the off-season and you will start out stronger in the spring.
I can't speak for training at home on a trainer because I don't do that much, now that I have the spinning option.
Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived." Captain Jean Luc Picard