It's often easier to add to something that already exists than to start something new from scratch. And when you're the one who created the thing in the first place, it's that much easier to make the improvements.
Even if the initial thing is so terrible it needs to be redone completely, at least you figured out what you don't want. All while getting into the habit of doing the thing.
I stumbled upon this when trying to get started with my master thesis. In frustration, I threw a bunch of buzzwords and random thoughts at LaTeX and then stashed it for a day or two. After that, I picked it up, looked at it critically and went "well, this paragraph is obviously wrong, here's what it SHOULD say..." and two hours later I had a very rough draft finished.
Write out the whole thesis as headers and fill all the sections with Loren ipsums until you have the page count you want. Then, start filling in the real text
Even if the initial thing is so terrible it needs to be redone completely, at least you figured out what you don't want. All while getting into the habit of doing the thing.