Distracted writing is often not good writing. As a consequence, a number of people have gone out of their way to develop software that makes it a fair amount more difficult to be sidetracked by the countless things that aren't word processing software available on any computer, regardless of platform. Here are a handful of them, where to find them and how they work:
FocusWriter is a full-screen word processor intended to make the creative process as immersive as possible by making your typing appear at the very front of the screen, surrounded by nothing but a customizable theme or background. It offers a handful of tools and customization options only visible when your mouse is at one particular point on the screen, which then goes away when your mouse moves. The software provides real-time statistics about how much you've written and allows you to set daily goals in terms of word count or time spent typing, along with day-to-day analysis of how much work you're getting done, making apparent how productive you have or haven't been. It also conveniently handles several auto-save procedures and allows for multiple documents to be open via tabs.
Freedom is a more general-purpose productivity application. It consists simply of a dialog box that one inputs a quantity of numbers into, which then determines how long the program will completely turn off the system's networking for. If you're doing research as you write, this obviously won't work out for you at all, but if you simply need an environment with less distractions (and it's that pesky internet that's doing you in), Freedom might provide you with exactly what you need in order to ensure that your inbox migrates toward your outbox.
WriteRoom is a MacOS and iOS-compatible full-screen writing application. It offers synchronization features so that your documents all stay consistent with multiple versions regardless of what platform from which you write them. Its developer seems not to recommend it for lengthy, sophisticated projects given that it does not have a huge feature set (and doesn't necessarily need one), instead recommending several other programs for long projects and ones that will require substantial technical editing; however, if you're using one of these programs, it's likely to manage the drafting process.
Q10 is a free, heavily customizable Windows-based full-screen text editor. It functions as a portable executable, even to contain drafts of your documents, making it easy to carry as an application from computer to computer via services like DropBox or your personal USB drive. It provides many statistics in real-time as you write, allowing you to see word count, page count, and character counts as they increase before your eyes. It also allows you to specify a formula to determine your document's page count, rather than relying on Word or another application's highly variable ones, which in turn makes it easier to know exactly how much you have finished before you start copying and pasting like mad. Q10 allows you to set targets, make notes within documents, check spelling, and set timers where necessary to tell you how much you write in a given period of time, for example.
OmmWriter is another full screen text editor designed to put an emphasis on concentration (perhaps why the company expresses most of its advertising and marketing through a lengthy video in which you can watch the program being used to advertise itself). It offers customization through fonts and backgrounds, as well as features like chromatherapy, which allows for soothing gradients to move as you work, as well as ambient noise and keyboard sounds intended to make the writing experience easier and less stressful. It feels a bit New Age, but for some, it may be just right.