It is true that if you distract your mind, it sometimes helps physically. I don't often feel like working out, but once I do, I feel better. Getting out of the house, even to browse around the yard (even at this time of year) helps my mood too. Being stuck in the house allows me to think too much about what I can't do.
I also believe in writing in a "Gratitude Journal". Yes, even we MS patients have people or things to be grateful for. If I concentrate on the abundance in my life, the things I don't have or can't do seem smaller by comparison...