The thing about writing is that it can be very easy to fall into the trap of “it has to be perfect when it hits the page.” When that doesn’t happen (because of course it doesn’t), that’s when many people give up.
The more sneaky trap, I think, is the one that appears for those who made it past the first one: “it doesn’t have to be perfect, but it has to be everything.” Maybe the words aren’t the right ones, that’s what second drafts are for, sure, but you can’t edit what isn’t there, so you better get it all down the first time around.
So a writer sits there and thinks “I really want to write a story about a duel, but before I can do that I have to write all the lead up to it, and I can’t do the lead up with out making sure I have parts of the B-plot going through it, and I can’t do that before I figure out how I’d reference the theme, and I can’t reference the theme without creating some parallels in some other scenes, and I don’t even know what those scenes are–”
And then nothing happens, and the screen remains blank, because there’s more than one way to try and get everything “perfect” before it even hits the page.
Don’t worry about the theme. Don’t worry about those side scenes. Don’t worry about the references you have to shove into places you haven’t even imagined yet.
Write the duel. Write the next thing you’ve got a clear picture of. Write it out of order, write it from the wrong character POV, write it in a different tense. You can’t edit what isn’t there – and you can’t write when you’re too afraid of getting it wrong.