Slate has a pretty interesting article discussing these two different writer problems. Truman Capote and Ralph Ellison are discussed, but I think what affected them most was the one hit wonder syndrome. They achieved unimaginable praise and validation from one work and then could never finish another because they were afraid it would not measure up to the previous work. They worked forever on the next project and died before it was finished.
I am a frequent procrastinator. I lose myself on the information superhighway, and don't resurface until it's 'too late' to write. I'm trying to be better, but it's an uphill battle. Why do I do this? Hard to say. Sometimes I'm just unmotivated. Or brain fried. It's also so sort of denial in my case. Don't know the true root of it.