For me it was advice from Dan Harmon: “Don’t try to prove you’re a good writer, you’ll never write anything. Try to prove you’re a bad writer and you’ll write everything.” Not perfect advice but it really does help me write when I’m being overly critical of my ideas.
I reached the stage of a (non-fiction) writing project where I had tons of notes, but no sense of what an eventual manuscript might look like. I discovered Zettelkasten and it was a revelation, not because I think it’s the only way to write, but because it was an answer to my precise problem of how to turn a ton of notes into a manuscript. I’m still a long way from being finished with my project, but I can get my pen moving every day and that in itself has been an enormous relief.
@professed @Dungeondaddyd20 I guess this is similar to other answers here (and yours in particular), is to avoid trying to write a big project from the get go. It’s much easier to try to write smaller more manageable stuff (which zettelkasten encourages). At some point you might be able to use the bits you’ve already written for a larger project.