You have to write a document. You have a rough idea. You do not have the exact structure. You procrastinate, waiting for the ideal form to emerge. The deadline arrives. You are still not done.
We have all experienced this. It could be a document, a report, or a presentation.
We believe we have to do things linearly, from start to finish, beginning to end. That is not the case. If you have the end hashed out, start with the last part. If you do not know how to start but have the rest of the content thought out, pen that down. You need not go from start to finish linearly; it can be all over the place, zigzagging.
Once you have something written down, the structure starts emerging. You get ideas. The brain fog clears.
Try this iterative, non-linear process the next time you are stuck.