C. S. Lewis once defined the nature of addiction: an ever-increasing addiction to an ever-decreasing pleasure, or something to that extent.
What if I happened to come up with something good, a good thought? The pleasure it brings is getting smaller and smaller all the time. The doubts are increasing. The pessimism is deepening, the general feeling: sigh, what shit my life is, what a loser I am, how useless this all is. :)