Lol, I am kind of dreading winter coming because it is just so freezing cold and dark and rainy here. I tend to suffer from a little bit of SAD so I know how you feel about lacking motivation. Here's what I've done:
Drink plenty of tea with caffeine regularly, as they do here, to keep your energy up and warm up your insides, so it's not so unbearable to face going out. Tea also makes you less hungry.
Also take a multivitamin every day -- I take Centrum Performance, which has extra things in it that keep your energy levels up and increases concentration and alertness. I also recommend Well Woman, which is a multivitamin specifically designed for women. You just feel better in general.
If possible, even consider investing in a UV lamp, which apparently erases the sluggishness that comes with lack of sunlight -- I've never tried it, but I hear it really helps.
If you continue to workout regularly from this point forward and remember your goal of staying fit and trim, and keep reminding yourself that you
really don't want to go back to the way you were just because it's winter, you will be so naturally in the habit that you won't really want to give it up. And don't let the colder weather stop you from walking everywhere... just bundle up and go for it. Remember, action precedes motivation.
Also, remember to keep eating healthy. The colder weather in general makes people crave fatty comfort foods... if you can resist temptation and eat plenty of wholegrains, fresh fruits & veggies and protein to satify your hunger, you will do fine.