I'm an art director and also spend most of the day sitting. It's no excuse.You have to make time to get off your butt and move. It's what our bodies are intended to do. I also play hockey, so it's very important I maintain my weight. My metabolism problems are definitely not a fun problem to have, I assure you.
Sorry you have trouble keeping weight on. Since most of us have to go "low carb" to keep the weight off, what happens if you do the opposite? Try baking yourself a loaf a fresh bread every day.