Mosaic from the Villa Romana del Casale, Piazza Armerina, Sicily. Third century CE.

LXXVIII — On the healing power of the mind

But do not of your own accord make your troubles heavier to bear and burden yourself with complaining. Pain is slight if opinion has added nothing to it; but if, on the other hand, you begin to encourage yourself and say, “It is nothing, – a trifling matter at most; keep a stout heart and it will soon cease”; then in thinking it slight, you will make it slight. Everything depends on opinion; ambition, luxury, greed, hark back to opinion. It is according to opinion that we suffer. A man is as wretched as he has convinced himself that he is. 

Seneca Epistles 66-92, by Lucius Annaeus Seneca the Younger. Translation by Richard Gummere. Loeb Classical Library.