It is a huge challenge to write while depressed. Every thought undercuts the previous one. I am able to find the futility of every idea that comes along so that it is nearly impossible to construct the foundation of a big, interesting, subject.
Oh, there are plenty of ideas running around, but they do not obey my command to neatly form a line. They talk out of turn and are generally rowdy. When feeling down, some thoughts bully others into accepting that there is no use in trying. Remember that time he actually thought he could accomplish something, and he failed? The bully chuckles. You see, there are others who wish to speak, who can remember when trying something difficult worked out, but they have now become quiet and sheepish.