Here's a writing approach that can help you shut down your internal editor, increase your output and focus your concentration. I call it The Essay Test Technique.
Choose three subjects. Set a timer for 30 minutes, and in three 10-minute sessions, pound out an essay draft on each of those three subjects.
Your goal in that time isn't to sketch out notes or put down bullet points. Rather, it's to write complete works in circumstances just like what you faced during those dreaded essay tests in school. The time pressure forces you to ignore your internal editor, so you'll write more freely. There's no time for your brain to censor your thoughts, or force you to go back and reword things. You have no choice but to write freely and fast.
And don't forget the best part of this technique: there's nobody grading you!
In ten minutes of hyper-focused writing it's possible to write as many as 200-300 words--or more, depending on your typing speed. Even a relatively slow typist (say, 25 wpm), can hammer out 250 words in ten minutes. This means you could flesh out a brief scene in a novel, a good-sized chunk of dialog or even a complete blog post.
When you simulate the pressure of an exam where you have a fixed (and usually insufficient) amount of time to write something that must be handed in when time runs out, it forces you to concentrate and do the best you can with the time you have.
If you have more endurance than I do, consider setting the timer for a longer period, say 60 minutes, and shoot for writing six drafts. You might get into a good rhythm and generate several freely-written first drafts.
Keep in mind that everything you write using this method will need further editing to make it perfect (all writing does!). But this technique should help you focus your mind and generate a massive amount of quality first-draft copy in a short amount of time.