"Otherwise, we who are in the know, we lone voyagers, we nympholepts, would have long gone insane."
Humbert is talking about people like him, who are attracted to young girls. He mentions that not every young girl is a nymphet, which keeps men like him from going crazy. Humbert uses three ways to describe people in his situation. Nabokov also uses adjectives and qualifiers for every noun except for nympholepts, his own word.
He gives the feeling of a small, elite class of people. He is not condescending to his kind but makes them seem noble- especially in the sense that people are born into the condition of liking young girls. It is more like predestination, not a choice. These lone, knowledgeable men seem like they had this fate bestowed upon them in an archetypical hero's journey.
This select group is lonely. Humbert is revealing his plight and evoking sympathy from his audience. We read his words, and thus see the world from his view. We do not see him as a petophyle, we see him as someone stricken with an elite disorder. Humbert has to hide his desires from the public to be deemed acceptable. This mental prison mimics the fact that Humbert is writing in an actual prison. By writing this work, Humbert liberates himself from a psychological prison, but ironically has entered a physical one. He went to prison because of murder, not because of his nymphetism. He is put in prison from an act of free will, not from his inherent psychological longings.