I'm much better these days, but its taken years to get here, and sometimes I slip back into the habit of tweaking things here and there as I write.
I'm good with flash fiction, most pieces I write call for no more than 200 words, so I'll set my timer and just keep writing until I get to the end of the story, or the alarm sounds. I've usually overwritten by 150 words, so I'll take a hatchet to my work, and par the story down to the required length. This takes two, maybe three edits until I am satisfied.
Longer works prove more tricky, I have to summon up the willpower not to start fiddling. Re-reading, changing a sentence here and there. Realising I need to add another two sentences to get things to make sense now, because I've changed something crucial. So I add a clue, remove a character, change tense, delete dialogue and before you know it I'm screaming and pulling my hair out, three hours have passed and I've only written fifty words! Sound familiar?
If you are constantly worrying about spelling, syntax or tense, then you're interrupting the flow. Stop it, and worry about revisions in the editing stage later. Just get the story out on the page.
What do you think? Can you resist the urge? Or do you get along fine editing as you go?