Dupes indeed are many: but, of all dupes, there is none so fatally situated as he who lives in undue terror of being duped.

Thomas Carlyle (1795-1881) Scottish essayist and historian

Lecture (1840-05-22), “The Hero as King,” Home House, Portman Square, London
    (Source)

The lecture notes were collected by Carlyle into On Heroes, Hero-Worship, & the Heroic in History, Lecture 6 (1841).