I am entirely unqualified to comment on the Italian, or the accuracy of the translation -- as a singer, I should probably have learned Italian at some point, but while I'm able to pronounce it, I usually have only a vague idea what it means. (The translation below's from my CD booklet, by Clara McTigue and Will Power.)
To get the full effect, hear Monteverdi's setting of this from his eighth book of madrigals. The last line is one of the best text settings I know. Or hey, print out the score and perform it yourself!
Hor che ’l ciel e la terra e’l vento tace,
E le fere e gli augelli il sonno affrena,
Notte ‘l carro stellato in giro mena,
E nel suo letto il mar senz’ onda giace.
Veggio, penso, ardo, piango e chi mi sface
Sempre m’è in anzi per mia dolce pena,
Guerra è il mio stato d’ira e di duol piena
E sol di lei pensando ho qualche pace.
Così sol d’una chiara fonte viva
Move ‘l dolce l’amaro ond’ io mi pasco
Una man sola mi risana e punge
Perchè ‘l mio martir non giunge a riva,
Mille volt’il dì moro e mille nasco,
Tanto da la salute mia son lunge.
Now, while sky and earth and wind are still,
and animals and birds are locked in sleep,
and night leads her starry chariot about
and the waveless sea reposes on his bed;
I watch, I burn, I agonize and weep; and he who wastes me
is ever before me, source of such sweet sorrow;
my heart is in turmoil, full of wrath and grief,
and only thoughts of him bring some relief.
Thus from a single, pure and vigorous source
flow both the sweet and the bitter than comfort me;
a single hand both heals and pierces me;
and, since I cannot see an end to anguish,
a thousandfold each day I die, a thousandfold I am born:
so far am I from my well-being's source.