I always worry about whether or not I'll be able to complete a fic. I haven't attempted that many multi-chapter stories, but I've never finished any before. So if I get a nice groove with a story and am updating fairly frequently, if I have a day or two where I just don't feel like writing I will completely flip my lid convinced that I am never ever going to be able to write for this story again if I can't write something good right this instant.
