Then again, now I think about it, it's simply a matter of discipline and allowing oneself to write less than perfectly. Or just crap. For a while I was able to do that, to just write every day even if it was crap. I used to joke with my writer pals that my WIP was tentatively and affectionately entitled Crap, the Novel (CTN). A couple of years later, CTN is no longer called CTN, and I've lost that spirit of "just write, even if it's crap." I've gone back to trying to act all writerly again, and it wasn't working.
Let's face it: It's not really that hard. I have the story in my head already. I know where the plot is going. I know these characters as if they were real. So what is the problem? So what if my prose comes out flat and unattractive? That's what rewrites are for. I can't rewrite unless I have words on the pages, not in my head.
And think about it this way: 500 words a day isn't really that much, especially if "quality" isn't the ultimate criterion. 500 words a day means 185,500 words in 365 days. That's practically two or three novels in a year, or one epic son of a bitch. Even if half of it was utter crap, and there's a good possibility that it is, I'd still end up with 92,000 words of material. That's one novel a year... instead of half a novel in four years.
Think about it. It's only a matter of Math (and yes, I know Math isn't everyone's favorite subject). You don't have to be too ambitious. You don't have to write 1000, or 2000 words a day. Just 500. That's a very attainable and sustainable goal. The tough part is "500 words, every day." That's where discipline comes into play. It doesn't matter if you're sick, tired, busy, or on vacation. 500 words a day.
Does it still sound that impossible?