“Our doubts are traitors, and make us lose the good we oft might win, by fearing to attempt.”
Doubt becomes harmful when it stops useful action before reality has had a chance to test what might be possible.
Source details
Measure for Measure, Lucio to Isabella. The retained modern locator is Act I, Scene 4; the represented 1623 First Folio witness, reviewed through the 1910 Methuen facsimile transcription on Wikisource, labels the same scene Act I, Scene V (Scena Quinta). Lucio urges Isabella to test her ability to plead for Claudio rather than let doubt prevent the attempt. This is fictional dramatic speech, not Shakespeare speaking autobiographically or a universal psychological law.