Another mistake we make is to assume that what flows from our pen must be finished product. Logically, we know this makes no sense. There's always a bit of re-writing before the proofreading and editing. We would never expect others to deliver perfection without practice.
Whether it's the next chapter in your novel or a page of marketing copy for your website, it can help to sit down and intentionally scribble the ugliest, roughest draft you can imagine. Make it your plan to write something so simple, so messy, so basic, so ugly, that you can't possibly use it. This is just a note to yourself about what you're planning to think about considering writing.
This is much like the trick I use to get myself to do household chores. If a picture needs hanging, next time I see the hammer I lay it on the floor where the picture is to be hung. Then when I run across the box of nails, I set that in place. If the picture needs a hanger attached to it, that goes in the pile as well. Eventually I walk past, look at this instant picture hanging kit sitting on the floor, and realize that it will take almost no effort to finish the task. It gets done.
The hardest part about writing is writing. Not the polishing, the formatting, the editing. Just starting. Just putting down the few words that say what we really mean.
Prewriting is a way to start ugly and simple and just get something down on paper.
Once the task is started, sometimes the compulsion to continue is overwhelming.
That's okay too.