Motivating struggling writers can be quite a challenge. With struggling writers, it is important to create high-interest assignments that students will want to work on, assignments that they will want to put forth the effort on in order to succeed. If struggling students are bored with a writing topic, they won’t be as invested in it and won’t want to put the time and effort in to make it the best it can be.
Writing is a process. It is recursive. No piece of writing is ever "final." Something can always be better. I often feel this way whenever I read back over my own old essays and inevitably find a sentence that could be better, a paragraph that could be stronger, or a word that could be more precise.