It is a misfortune not to be loved at all, but an affront to be loved no longer.

[C’est un malheur de n’être point aimée ; mais c’est un affront de ne l’être plus.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Persian Letters [Lettres Persanes], Letter 3, Zachi to Usbek (1721) [tr. Healy (1964)]
    (Source)

Chiding Usbek for leaving her and his other wives behind as he travels to France.

(Source (French)). Alternate translations:

'Tis a Misfortune not to have been belov'd; but 'tis an Affront to be belov'd no more.
[tr. Ozell (1736)]

It is a misfortune not to have been beloved; but it is an affront to be beloved no more.
[tr. Ozell (1760)]

Not to have been beloved is a misfortune; but to be so no more, an affront.
[tr. Floyd (1762)]

It is a misfortunate not to be loved, but to have love withdrawn from one is an outrage.
[tr. Davidson (1891)]

Not to be loved is a misfortune, but to be abandoned is an -- outrage.
[tr. Betts (1897)]

It is a misfortune to be not loved; but it's an insult to be no longer loved.
[tr. Mauldon (2008)]

It is misery not to be loved, but it is an offense to be loved no longer.
[tr. MacKenzie (2014)]