For he says that evils are neither diminished by time nor lightened by being premeditated; that meditation on evil to come, or, it may be, on that which will never come, is foolish; that every evil is sufficiently annoying when it comes; that to him who has always thought that something adverse may happen to him that very thought is a perpetual evil; that if the expected evil should not happen, he would have incurred voluntary misery in vain; that thus one would be always in distress, either in suffering evil or in thinking of it.

[Nam neque vetustate minui mala nec fieri praemeditata leviora, stultamque etiam esse meditationem futuri mali aut fortasse ne futuri quidem: satis esse odiosum malum omne, cum venisset; qui autem semper cogitavisset accidere posse aliquid adversi, ei fieri illud sempiternum malum; si vero ne futurum quidem sit, frustra suscipi miseriam voluntariam; ita semper angi aut accipiendo aut cogitando malo.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Tusculan Disputations [Tusculanae Disputationes], Book 3, ch. 15 (3.15) / sec. 32 (45 BC) [tr. Peabody (1886)]
    (Source)

Discussing the teachings of Epicurus (fr. U444). Source (Latin). Alternate translations:

For that neither are Evils abated by long time, nor yet alleviated by foresight of them; and that the poring on Evils not yet come, and perhaps that never will come, is foolish. For that all Evil is Vexation enough, when it is come; but he that is always thinking that some Adversity may possibly befall him, to him it becometh an everlasting Evil; but if it shall never actually come upon him, a voluntary Disquiet is taken up on false grounds; so the mind is always vex'd, either with enduring, or expecting Evil.
[tr. Wase (1643)]

Evils are not the less by reason of their continuance, nor the oighter for having been foreseen; and it is folly to ruminate on evils to come, or that, perhaps, may never come; every evil is disagreeable enough when it doth come: but he who is constantly considering that some evil may befall him, charges himself with a perpetual evil, for should such eve never light on him, he voluntarily takes to himself unnecessary misery, so that he is under constant uneasiness, whether he meets any evil or only thinks of it.
[tr. Main (1824)]

For evil ls not diminished by time, nor alleviated by premeditation: that it is folly itself to brood upon evil that is future, or indeed, perhaps, is not to be at all: that evil is hateful enough when it comes: that, to the man, who is always musing upon that which is to come, his meditation itself becomes an eternal evil; and, should it prove that his apprehensions have been groundless, he burdens himself with a voluntary misery; and thus, between the encounter and contemplation of evil, he is always in trouble.
[tr. Otis (1839)]

Evils are not the less by reason of their continuance, nor the lighter for having been foreseen; and it is folly to ruminate on evils to come, or such as, perhaps, never may come; every evil is disagreeable enough when it does come; but he who is constantly considering that some evil may befall him, is loading himself with a perpetual evil, and even should such evil never light on him, he voluntarily takes upon himself unnecessary misery, so that he is under constant uneasiness, whether he actually suffers any evil, or only thinks of it.
[tr. Yonge (1853)]

Evils are not diminished by the passage of time, nor made easier by pre-rehearsal. In fact it is foolish to rehearse misfortunes which have not yet happened and which may not happen at all. Each of our misfortunes is distasteful enough, he says, when it is already here: those who have constantly been thinking about what disagreeable things are on the way simply make their evils perpetual. And those things may not happen at all, in which case all their voluntary misery goes for nothing. The result is that they are always in anxiety, either from the evils they undergo or from those they anticipate.
[tr. Graver (2002)]