You have two brains: a creating brain and a controlling brain. Both are good, but they don't always get along. The problem is that your controlling brain has been fed for years. It has been well educated by well-meaning teachers while your creative brain was left to wither, sick in bed.
To write well, and to find what you have to write about, you need to give your creative brain a workout. You do that by writing the first thing that comes to your head and refusing to control yourself, to edit. You can do this by hand, on a typewriter (my preference) or with a computer.
It's most difficult to exercise your creative brain with a computer because it's easiest to edit with a computer. But if you must use a computer to freewrite, here are a couple of tricks.
1. Turn off automatic spell/grammar check.
Who cares if you misspell a word? Your controlling brain does. Keep that guy out of your head by turning off those red and green squiggly lines. You can learn how to turn off the spelling and grammar check here for your particular flavor of Microsoft Word.
2. Watch the keys as you type.
This keeps you from watching your “terrible” words form on the page. By the way, they're not terrible. That's your controlling brain talking. They may not be eternal though. Instead, they are a means to finding your eternal words.
3. If desperate.
Change the color of the font to white or light yellow so you can't see what you're writing. To your controlling brain, it will be like you're not even writing. It will be as easy as breathing.
Do these three things and your controlling brain will have a hard time doing what it's best at: controlling. Give your creative brain a chance to catch up and try it.
Freewrite for fifteen minutes. Do NOT use your backspace key. If you feel the urge to use your backspace key, try the tricks above. If you still find a way to edit, switch to writing by hand.
Post the practice in the comments (spelling mistakes and all), and, if you like, you can use a pseudonym of your choice (maybe your favorite super hero).