If you can't bring yourself to stay outside for more than a few minutes in the winter, there are plenty of ways to get a great workout indoors, other than your same old gym routine. Try out some group fitness classes. There are many gyms and fitness studios that offer a variety of classes including boot camps, strength training, spin classes and yoga.
Yoga in particular can be very beneficial in the winter. It helps stretch and strengthen muscles, which can relieve chronic pain and gives you time to meditate and reflect, which can reduce insomnia. It can also lower blood pressure and give you more energy throughout your day, which is particularly helpful in the bleak winter months.