We easily forget our faults when they are known only to ourselves.

[Nous oublions aisément nos fautes lorsqu’elles ne sont sues que de nous.]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶196 (1665-1678) [tr. Stevens (1939)]
    (Source)

Included in the 1st (1665) edition, reading "When we alone know of our crimes, they are soon forgotten [Quand il n’y a que nous qui savons nos crimes, ils sont bientôt oubliés]." Another variant is "Nous oublions aisément nos crimes lorsqu’ils ne sont sus que de nous" (bringing in the "crimes" term rather "faults").

See also ¶327.

(Source (French)). Other translations:

We easily forget our Faults, when no body knows them but our selves.
[tr. Stanhope (1694), ¶197]

We easily forget crimes that are known only to ourselves.
[pub. Donaldson (1783), ¶74]

We easily forget crimes which are known only within ourselves.
[ed. Carvill (1835), ¶62]

We easily forget our faults when they are only known to ourselves.
[ed. Gowens (1851), ¶205]

We easily forget those faults which are known only to ourselves.
[tr. Bund/Friswell (1871), ¶196]

Failings known to us alone are easily forgotten.
[tr. Heard (1917), ¶201]

Our faults are easily forgotten when they are known only to ourselves.
[tr. FitzGibbon (1957), ¶196]

We easily forget our faults when no one but ourselves knows them.
[tr. Kronenberger (1959), ¶196]

Our misdeeds are easily forgotten when they are known only to ourselves.
[tr. Tancock (1959), ¶196]

We easily forget our faults when they are known to ourselves alone.
[tr. Whichello (2016) ¶196]

A number of translators link this to Seneca, Moral Epistle 3:

Most people fancy themselves innocent of those crimes of which they cannot be convicted.

Innocentem quisque se decit, respiciens testem, non conscientiam.