The truth is, most of us procrastinate because we feel overwhelmed by the task. But think about it. If you've made it through most or all of your graduate work, you have no doubt run into people with advance degrees that make you wonder how in the world they earned that MS or PhD after their name. Surely if they can do it, you can do it. You're not a fraud, and you're not incompetent. You would not have made it this far if that were the case.
But if you want to get it done (and you know you do), you will have to face some tasks that may not be all that enjoyable. Specifically, you will have to make a schedule for actually sitting down and writing. You may have to be quite strict with yourself, imposing structure on your schedule, because nobody is going to do it for you.
Break the process down into steps, even if those steps sound amazingly trite and stupid. For instance, your time block from one o'clock to three o'clock on Tuesday may say, "Come up with three possible titles for dissertation." If you have to sequester yourself in a remote corner of the library in order to do this, then so be it. And if you finish in one hour, spend the other hour taking a nice walk or enjoying a well-deserved cup of coffee.
Your next time block may be assigned to "Develop a thesis statement," and the one after that may be, "Spend two hours gathering source material." Of course, these specific tasks will differ depending on whether you're studying Native American folklore or positron emission, but most of the steps have correlating steps across the disciplines.
When it comes to writing your thesis, here's a little secret: you don't have to start at the beginning. In fact, if you force yourself to write the abstract, then the introduction, then each chapter in turn until you finally write the conclusion, you may be setting yourself up for a difficult time. If one particular experiment in your investigation was particularly meaningful for you, write that chapter first, even if you don't know what chapter number that will be. With each step you accomplish, you gain momentum. If you need to do the steps somewhat out of order, it's fine to do that and fill in the gaps later.
Whenever you come to a task that feels overwhelming, break it down into smaller tasks, even if those tasks seem idiotically simple. Work at the times of day (or night) you are most productive, and reward yourself when you finish an onerous task. You know what you have to do. Now quit procrastinating and do it.