When fatigued or frustrated, walk away. Time spent stitching under duress generally proves counterproductive for me. I always have things to straighten or organize if I am not ready to leave the sewing room or else, I cut the light off and do something else. Sometimes you just need to mentally digest things to work them through. By giving yourself time and space to do so, you can return to the project with a fresh perspective and new found energy. After all, when it ceases to be enjoyable, why do it?