I strongly second the recommendation for phototherapy. It made a world of difference to my younger son in our long winters. 30 minutes every morning while he drinks a cup of coffee and surfs the internet - voila! Cheerful and upbeat all winter long, where before he could be like a grumpy bear.
And as far as the work goes, make a plan for dealing with overloads, and see if you can distribute the travel to other members of the team. They might be thrilled to pick up some of that for you.
Stay sane, stay healthy, stay happy.