Keep breaking down the task into smaller steps until you can find something you can will yourself to do. Focus only on the next step, or you'll get distracted with planning. At times, when I was ill, the big task was "personal hygiene," but I had to break it down until my first step was "sit up." On one or two days, I even had to break that down further - "wiggle my toes". I learned this from an Arctic Racer who struggled with getting out of a warm sleeping bag to pack up her camp and continue the journey.