Try it for a week. Select a time each day, preferably not the evening unless you're a night owl, and write for 30 minutes, without email, reading or other distractions. Don't listen to the voices in your head saying you "should be getting more done," or "you should be writing more than this." I'll bet at the end of the week you'll be pleasantly surprised at your output, and pleased with the increasing ease with which you can sit down to write. You’ll start to see progress on your dissertation or article and maybe come to believe that you will finish one day.
Furthermore, don't forget about being accountable to someone. Let someone else know that you're going to be doing daily writing. Perhaps you can find a writing buddy, or someone in your dissertation group. Or join one of my coaching groups – our listservs allow for lots of accountability during the week! My membership site, CafeAcademia.com (stay tuned,) will have a place for finding writing buddies.
Don't forget, if you always do what you've always done, you'll always get what you've always got. Start setting yourself up for success starting right now!