He was focusing so much on the dread of writing and his self-doubt that it was inhibiting him from seeing his full potential in producing a great essay. The fact that he viewed the essays and writing as dull restricted him from seeing what the end result could look like with some effort.
He speaks about dreading the task of writing about a dull essay. He realizes as soon as the teacher began reading and his peers showed their interest in what he wrote that he had found his calling. He could