Don't shoot the messenger, but this is what I believe.
Just like you, I've sat down to a blinking cursor on a blank page and been terrified that this time I won't be able to do it. The words won't flow, the plot won't come, the characters are just going to stare at me, hoping for direction. And every time, once I get my fingers warmed up and I type two or three pages of crap that I have to write first, I'm able to get to the good stuff buried underneath. This is why I say there's no such thing as writers block.
It's actual name is procrastination.
And the actual name of procrastination is fear.
We put off writing because we're afraid of the blank page, the dormant characters, the crappy dialogue that we're sure to produce. We're afraid of the plot kink that hasn't quite worked itself out yet, so we put off having to deal with it by jumping on Twitter, checking out Facebook, or surfing Tumblr for just a few more minutes.
I always tell new writers that they can't be afraid to suck, but I think that established writers need to be aware of that as well. Sucking is part of the process. No one has ever produced a first draft that anyone other than their mom would praise.
We ambush ourselves with self-doubt, whittling away precious minutes with ultimately useless activities, then tell ourselves that we'll write tomorrow. Tomorrow there will be more time. But the single paragraph that you write tonight puts you one paragraph ahead for tomorrow, and the three sentences you tack onto it during your lunch break gives you a head start on the evening. All those short paragraphs written in stolen moments, and choppy sentences you forced out of yourself before going to bed can be massaged into a coherent narrative when you're editing.
So don't be afraid.
Write the words.