Lord, have mercy.
I've had this problem many, many times. I've found that if you pay attention it's part of a process. If I was going to start a paper, I knew beforehand to at least get semi-started far in advance and take all the time I needed. I'd punch out 3 or 4 pages, break for a few hours doing whatever, then repeat. I'm assuming that you work, as I did, so I always took that into account as well.