When you come to actually write, the validation narrative is often what gets you stuck. The reason your paper is never good enough to send off is not because you don’t have something to say but because you think of the process as validating your worth. The content is in many ways incidental to the fact of having a publication in a high ranking journal.
Focusing on what you want to say, and to whom, can be a powerful road out of the stuck. It also helps you see that some media are more suited to particular material than others.