Don't get discouraged. Where I live, the winter has been simply dreadful. Normally winters aren't too bad, but this one has had snow, slush, and freezing weather *far* more days than usual. It's really cut into my outdoor walking time and consequently I've largely "plateaued" this winter too.
When you hit a plateau like that, it helps to gradually up your exercise a little bit, a week at a time. Try an extra 15 minutes a day for a week, then work up to an extra 20, and then gradually up to an extra 30-45 minutes. As you exercise more, your body's fundamental metabolism will shift.
What also works for me is to restrict complex carbs (whole wheat bread, non-instant oatmeal) to only *one* meal a day. For me that meal is breakfast. If I'm "being good," the rest of the day the only carbs I eat are leafy greens and low-glycemic vegetables (green beans, asparagus, cucumbers, spinach, peppers, etc.) and low/mod-glycemic fruits. Tropical fruits (bananas, pineapple, mango etc.) are out. I also have to avoid entirely starchy vegetables like corn, carrots, potatoes.
I know that when my weight loss slows, it's because I'm eating rice with dinner, or too much fruit, or breaking down & eating Girl Scout cookies or something like that. It takes a *total* lifestyle change to eat that many vegetables in my low-glycemic meals.