So live, my boys, as brave men; and if fortune is adverse, front its blows with brave hearts.

[Quocirca vivite fortes
fortiaque adversis opponite pectora rebus.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Satires [Saturae, Sermones], Book 2, # 2 “Quae virtus et quanta,” l. 135ff (2.2.135-136) (30 BC) [tr. Wickham (1903)]
    (Source)

Often misattributed to Cicero.

(Source (Latin)). Alternate translations:

Live bravely then,
And in all troubles quit your selves like men.
[tr. A. B.; ed. Brome (1666)]

Then live Resolv'd, my Sons, refuse to yield,
And when Fates press make Constancy your shield.
[tr. Creech (1684)]

Then be not with your present lot deprest,
And meet the future with undaunted breast
[tr. Francis (1747)]

Bear up then, Boys! and stem the adverse tide,
Patience your stay and providence your guide!
[tr. Howes (1845)]

Wherefore, live undaunted; and oppose gallant breasts against the strokes of adversity.
[tr. Smart/Buckley (1853)]

So, then, live bravely on, and bravely stem adversity's opposing stream.
[tr. Millington (1870)]

Then live like men of courage, and oppose
Stout hearts to this and each ill wind that blows.
[tr. Conington (1874)]

Live, then, as brave men, and with brave hearts confront the strokes of fate.
[tr. Fairclough (Loeb) (1926)]

SO LIVE BRAVE LIVES: STAND UP TO THE BLOWS OF FATE!
[tr. Palmer Bovie (1959)]

So then, live, live and endure.
Meet life's difficulties with strong, enduring hearts.
[tr. Fuchs (1977)]

Good reason whereby you should be
happy and confront adversity
with an undaunted soul.
[tr. Alexander (1999)]

Live as brave men,
then, standing chest to chest with changeful fate.
[tr. Matthews (2002)]

So be brave
and bravely throw out your chest to meet the force of fate!
[tr. Rudd (2005 ed.)]

So live bravely, as men
With brave hearts do, and confront the vagaries of fate.
[tr. Kline (2015)]