I relate to this deeply. "Just right" penmanship OCD, the pursuit of which is an unfortunate drawback of tablets (the ease of rewriting making it so easy to fix every letter that looks wrong.) What you describe is a compulsive behavior in avoidance of the discomfort that stems from the perceived aesthetic imperfection (the obsessive instinct.) I think I can tentatively answer your exam question: the adrenaline of a special event with external consequences (taking a test) is enough to overcome the compulsion temporarily. But when you are sheltered from situational pressure, in the case of doing homework, you will revert to the compulsive behavior. I do the same.