Write for time goals, not words or pages. Set a timer for 20 minutes and work hard for 20 minutes. Then take a break, go do something else, or do the cycle again. Knowing that you get to stop after 20 minutes will help you stay focused and productive.
Let's say you went to the gym and you asked your trainer, how long will we work? And your trainer said, "Until you're exhausted." Ugh.
When you sit down to work "all day," that's what you're doing to yourself. Use a timer. Get more done.