When I leave London in early December, there’s a damp chill in the air that seems to penetrate right through my heavy coat. As someone who suffers from seasonal affective disorder (SAD), the onset of winter is always a tricky time. So when I step off the plane on the Caribbean island of Saint Lucia to be hit with a wall of warm, humid air and the earthy smell of rain-drenched vegetation, my mind feels immediately lighter. I haven’t come to the holiday hotspot just for a bit of winter sun, however. To really quash my seasonal descent into depression and anxiety, I’m joining an art retreat at be…