“Brave men are brave from the very first.”

The origin of this quote is often misattributed to Cicero; however, it is from Line 135-136 of Book 2, Satire 2 by Horace, "Quocirca vivite fortes, fortiaque adversis opponite pectora rebus." The English translation that most closely matches the one misrepresented as Cicero's is from a collection of Horace's prose written by E. C. Wickham, "So live, my boys, as brave men; and if fortune is adverse, front its blows with brave hearts."
Misattributed

As quoted by Macrobius in Saturnalia, Book VI, Chapter I

Book II, Satire II, Line 135-136 (trans. E. C. Wickham)
Satires (c. 35 BC and 30 BC)

Source: Pompeii

Book IV, ode ix, line 25
Odes (c. 23 BC and 13 BC)

William Cowper Prime in The Old House by the River (1853); first misattributed to Hawthorne in Notable Thoughts about Women: A Literary Mosaic (1882) by Maturin Murray Ballou, p. 239
Misattributed

Pure

De Providentia (On Providence): cap. 5, line 9
Alternate translation: Fire is the test of gold; adversity, of strong men. (translator unknown).
Moral Essays

Quentin Durward (1823), Ch. 3.