I'd reiterate all the posts talking about making writing a habit and simply writing every day to keep up the pace, even if it's only 250 words a day. This is what really made the difference for me. I have a terrible time knuckling down and actually writing and when I manage to write it's usually a short burst of 'gee this is fun' followed by 'it's sh*t' and 'oh, shiny'. Too many shiny new ideas, too many whispers of mediocrity on my shoulder, too many cries of 'no one cares' and 'it sucks' from my nasty critic-editor-brain.
The only time in my life I've been able to even temporarily silence those was when I simply decided to write a novel from start to finish, no editing whilst writing, no mucking about, just go. With a goal of 1000 words a day it took me three months to pound out 64,000 words. I missed my goal on more days than I hit it, but I still got something on paper. I picked a genre I am interested in but not my tops and I just started writing. I did this to make accepting that it was my first novel and was going to suck even easier to acknowledge.
Every bit of advice that says 'just accept that it's going to suck' was well in place and I didn't care, I just plowed on through. And viola, I wrote a novel. It's sh*te. It's in a genre I'm fine with never researching or reading again. But I still did it. I likely won't bother editing it because it's frankly terrible, but at least I reached the ultimate goal, getting it done. The first draft at least. I felt bad on the days I missed my word count goal and felt terrible about missing days entirely.
I tracked my progress with a spreadsheet and found something really interesting. There is a secret trick to writing a longer work. Want to know what it is? Momentum. The more days you write in a row, the easier it is to write; the more days you don't write in a row, the easier it is to not write. It was like watching a runner trip and stumble. Everything would go fine, then a little slip (a low word count day) would cascade into a stumble (a missed day), which would snowball into a fall (a missed weekend or three days). And when I finally picked myself up and started again, the word count was terrible on those first days. But if I didn't let that get to me and just kept at it, I would hit my stride again and get back to business.
It really reminded me of Sisyphus, from Greek myth. He tried to trick Hades into immortality and so was punished by being made to roll a massive boulder up a hill every day. At the end of the day he'd almost reach the crest of the hill, the boulder would roll back down, and he'd have to start over the next day. Writing is like that because you have to push a boulder up a hill, but writing is easier because if you just keep pushing, the boulder can't roll back down. It's far, far harder to get started than it is to keep going. Momentum.
So just start; you can edit later. Just write; every word gets you closer to 'The End'. Don't beat yourself up, some days you'll hit your goal, others you won't. But in the end you won't remember hitting or missing daily goals, they won't matter because you've got a finished first draft in your hands. Break up a big, seemingly impossible task into small, more manageable tasks. Writing a whole novel is hard; writing even 500 words a day isn't. Once you get started, don't stop. You'll want to take days off, don't. You'll take days off anyway, don't worry about it, just be sure to write the next day.
EDIT: The question of keeping attempted scenes is one reason I love Scrivener. I just drag and drop the scene into an "Attempts" folder within the file. They're still there and I can see them and copy-paste from them freely if I remember a well done line I'd like to keep. No separate files and no chance of losing them. But always make backups. Yikes, that mistake hurts.
