Many writers think of writing as rather like bricklaying, with the goal of positioning each piece correctly until the wall is complete. This mindset can shut you down every time you're not sure what the next word should be, or where it belongs. Train yourself to picture writing more like panning for gold in a river of words. The more words float past, the more likely you are to find what you need. Release the pressure to do it right. Write quickly, let go of the bad parts and keep the good.
A specific way to train yourself to focus more on quantity than quality is the practice of timed freewriting. Set a timer for a certain amount of time and write without stopping until the timer goes off. For this to work, it is critical to keep your fingers moving. Don't let the stream stop, even if you have to write the same word over and over, or write a string of incoherent words. It's often surprising what surfaces as you continue.
Not knowing what to do next creates an instinctive urge to escape into some other activity. You can avoid vast chunks of procrastination by training yourself to spend time defining a specific, visible next step instead of diving into comforting distractions. One method is to ask what is holding you back, or where you lack clarity. Write out 10 potential answers as fast as you can, regardless of how useful they are. If none of your answers help, generate 10 more.
A looming deadline has mysterious power to push you back into work you didn't feel like doing. Maybe your stifling self-evaluation takes a backseat to the fact that this just has to get done. Maybe it's the adrenaline rush. Either way, committing to a deadline will set a different pace and tone for your work. Make deadlines more real by making them public, or by giving a close friend some money and telling her not to give it back unless you show her a finished piece of writing by a certain date.