You just want to write a shitty first draft. The shittiest first draft on paper is superior to the most perfect first draft in your head.
Think of it as evolution. You just have to get it to the mammal stage the first time around. Actually it just has to have four legs and not swim in the water. So once you get your draft out of the primordial ooze and onto dry land, then you can take the time you need to turn it into a primate.