What is a first draft actually for?
Finding out what the thing is. Not producing publishable prose — which is why treating it as a performance is the most reliable way to never finish one.
The core insight. You cannot revise what does not exist. A first draft's only obligation is to exist completely, so that the real work — which is revision — has something to act on. A perfect first chapter attached to nothing is worth less than a bad complete draft.
Why writing and editing conflict. They are genuinely different activities using different modes of attention. Generating requires momentum and tolerance for uncertainty; evaluating requires scepticism and attention to detail. Running both simultaneously means each interferes with the other, which is why people stall three chapters in, rewriting the same pages indefinitely.
What a first draft frequently teaches you:
What the story is really about, which is regularly not what you intended. Themes emerge from writing rather than preceding it.
Which characters matter. Minor characters take over; central ones turn out to be inert.
Where the structure fails, which is only visible once there is a whole shape to look at.
That the beginning is wrong. Extremely common — the real starting point is frequently several chapters in, which you cannot know until you have written past it.
Practical approaches:
Write forward. Note problems and continue rather than fixing them. A bracketed placeholder for a scene you cannot face is better than stopping.
Lower the standard deliberately. Anne Lamott's phrase about accepting terrible first drafts is quoted constantly because it addresses the actual obstacle, which is not ability but embarrassment.
Finish before showing it to anyone. Feedback on an incomplete draft addresses problems you may have solved by the end.
Rest it before revising, so you can read what is on the page rather than what you intended.
The honest caveat. Some writers genuinely do work by revising as they go and produce near-final prose slowly. That is a legitimate method rather than a failure to follow advice — the point is to know which you are, not to follow the more fashionable position.