When you write easily, you always think you have more talent than you really do.

[Quand on écrit avec facilité, on croit toujours avoir plus de talent qu’on n’en a.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 23 “Des Qualités de l’Écrivain [Of the Qualities of Writers]” ¶ 45 (1804 entry) (1850 ed.) [tr. Auster (1983)]
    (Source)

(Source (French)). Alternate translations:

He who writes with ease always thinks that he has more talent than he really has.
[tr. Calvert (1866), ch. 15]

When anyone writes with ease, he always believes himself to have more talent than he has.
[tr. Lyttelton (1899), ch. 22, ¶ 13]

The fluent author always seems to have more talent than he has.
[tr. Collins (1928), ch. 22]