Most of the experienced writers advise that writing a draft and editing it should be a separate process. It’s true, and I’ve experienced it time to time.
Creative work is a different mind state than correcting an already drafted writing. In a recent TED talk, the speaker detailed the differences in brain activity during these two activities. It’s perfectly reasonable.
During creative work, when someone is in the flow, the brain areas responsible for creating are highly active. But the ones responsible with the perception on how good the thing one is creating is shut off. This means that the less you care what people will think about your work, the more creative and in-the-flow you’ll be.
Editing on the other hand is an analytical process. You scrutinize every sentence and assess whether they are the best possible, or how can you amend them. It requires the opposite brain functions then writing it in the first place.
That’s why you should take the writers’ advice and separate your workflow. It is also easier to do it this way.