When you sit down to write a new story--or even informational articles or emails, for that matter--you are trying to tap into the creative half of your brain.
Look at the image below:
On the right side, you have the places from which the writing flows. The right side of your brain contains your personality, the "you" that you're putting into your story, article, or email. It also contains the intuitive stuff, the part that tells you "this is good for the writing while that is not". The artistic, creative half of your brain--the right side--is the part that details your story as you sit down to write.
It can be tough to tap into the right half of your brain, particularly when most of us use the left half of our brain for work. The left side contains all the language, the writing skills, and the analytical part of our brains that examine what we write so critically.
So, when you sit down to write something new, you have to switch gears from the left side of your brain and start using your right hemisphere. It takes a while for you to get into the swing of things, but eventually you've shut off the left half of your brain as much as possible in order to let that creative right half get to work.
And then you make the mistake of trying to edit your work as you write. You look over what you just wrote for mistakes, evaluating your writing style, dialogue, etc. to see if you can make it better.
Big mistake! When you make that switch from creating to editing, you shut off the right half of your brain and hand control back to the analytical left side. When you try to start writing again, it will be hard to shut off the left side to let the right side take control once more. The more you switch back and forth, the more limited your right brain will be.
My advice--and the advice of greats like Stephen King: