Elizabeth Bishop takes a no-bones-about-it approach to the fear in "The Man-Moth." She doesn't hide the fear under metaphors, she just comes right out and tells us—multiple times—that the Man-Moth is scared. Of course, there are two options when you're scared: you can run away, or you can face your fears. Neither choice makes the fear go away, but which way we go does say an awful lot about our title character.
The Man-Moth's fear is less about the dangers involved in what he must do to reach his goal, and more about what he's going to do with himself once he succeeds. He isn't afraid of failure, he's actually afraid of success. Woah.
Fear is the driving force behind the Man-Moth's existence. He is so used to being in a constant state of anxiety that, if his current fears were completely alleviated, he'd just find something else to worry about. Chill out, man (-moth).