No time can efface the cruel ideas of a man's own foolish conduct, or of affronts, which cowardice or impudence has brought upon him.
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“Who, on the other hand, is not deeply mortified with reflecting on his own folly and dissoluteness, and feels not a secret sting or compunction whenever his memory presents any past occurrence, where he behaved with stupidity of ill-manners? No time can efface the cruel ideas of a man's own foolish conduct, or of affronts, which cowardice or impudence has brought upon him. They still haunt his solitary hours, damp his most aspiring thoughts, and show him, even to himself, in the most contemptible and most odious colours imaginable. What is there too we are more anxious to conceal from others than such blunders, infirmities, and meannesses, or more dread to have exposed by raillery and satire?” source