Misery is almost always the result of thinking.

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

Pensées [Thoughts], ch. 5 “Des Passions et des Affections de l’Âme [On the Soul],” (1850 ed.) [tr. Collins (1928)]
    (Source)

I could not find an analog in other translations of the Pensées, or in the published French.