Here Conrad uses a technique straight out of the novel: he makes a remarkable statement, then decoys us in the next paragraph with a surface explanation for it, namely that his anxiety was due to having run dry creatively. It is not a very convincing decoy, since he says that the creative dry spell caused him "some concern," a far cry from "the most anxiously meditated" creative project of a great artist's most fertile period. What he does not say outright, but we may infer, is that the anxiety was really caused by the colossal scale of his chosen task, involving the greatest possible heights of artistic subtlety towards the expression of a vision embracing the essence of the human experience.