In my case, it used to be perfectionism that shut down the creative process before it started. I spent nearly five years writing and rewriting the same novel chapter because I was terrified of having others see it and judge it.
Then - as I got less perfectionist - my biggest barrier became time management. I had to learn to aggressively get rid of unimportant and even semi-important activities to clear lots of time for writing and my indie publishing business.
Nowadays, I'm not very perfectionist, and I also have more time. But I still have to watch myself because it is easy to say "yes" to requests I really should say "no" to, and even short digressions for nonessential tasks turn out to be surprisingly big drains on my time, attention, and energy. So I really work to keep those to a minimum.
I'm also working on becoming a more efficient writer -- a.k.a. "boosting my writing tempo," as I discuss in Chapter 5 ("Optimizing Your Writing Process") of The 7 Secrets of the Prolific. Specifically, that means taking shorter breaks and extending the amount of time each day I can do bookwriting and other creative work. I mostly max out at three hours of such work before I get tired, although once in a while I can make it to five or six hours. (I spent another few hours a day doing admnistrative work related to my business.) I would like to do five or six hours consistently.
Even though invidious comparisons are perfectionist (Chapter 2), I do envy writers like Hemingway, who supposedly could sit at his typewriter for 8 hours. But I wonder how much of that time he really was engaged with his work. (Maybe someone reading this knows, in which case I hope you'll tell us!) I also have this notion that "typewriter time" was less dense, creatively, than "computer time." At the very least, there were some physical actions, like pounding on manual keys and inserting new sheets of paper, that broke up the monotony. On a computer, you're basically keyboarding (and on an easy keyboard) nonstop, so the process itself is more monotonous.
Everyone's process is different, of course--so please share yours. I'll also look forward to hearing about your process, your barriers, and your thoughts on Hemingway and the keyboarding question.