It's perfectionism. It had me in its grip in my high school French class for a bit, a course in which I generally excelled. I had to write a paper in the French language and, well, I couldn't start. I think if we experience some success in a subject, we begin to, as you said, set the bar higher. In my case, writing was also the area I felt I had expertise. Just remember, the aptitude is still there. And one paper may be excellent, the next three may be so-so. Don't let your performance on any one paper or even several define your level of competence, if that makes any sense.