Source identified1
“the moment you doubt whether you can fly, you cease for ever to be able to do it.”
Inside Barrie's fantasy narration, Peter's ability to fly is tied to undivided belief in that ability; as a quotation, the line works as a literary image of doubt interrupting action rather than as a literal psychological law.
Source note: Peter Pan in Kensington Gardens, Project Gutenberg #26998. The narrator comments on Peter after he feels shunned by the birds, explaining that losing faith in his power to fly would itself end that power within the story's fantasy logic. The line is narrator voice, not an autobiographical claim by Barrie.