Approach the easy as though it were difficult, and the difficult as though it were easy; the first, lest overconfidence make you careless, and the second, lest faint-heartedness make you afraid.

[Lo fácil se ha de emprender como dificultoso, y lo dificultoso como fácil. Allí porque la confianza no descuide, aquí porque la desconfianza no desmaye.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 204 (1647) [tr. Fischer (1937)]
    (Source)

(Source (Spanish)). Alternate translations:

What is easie ought to be set about, as if it were difficult; and what is difficult as if it were easie. The one for fear of slackening through too much confidence; and the other for fear of losing courage through too much apprehensiveness.
[Flesher ed. (1685)]

Attempt easy tasks as if they were difficult, and difficult as if they were easy. In the one case that confidence may not fall asleep, in the other that it may not be dismayed.
[tr. Jacobs (1892)]

Undertake the easy as though it were difficult, and the difficult as though it were easy, so as not to grow overconfident or discouraged.
[tr. Maurer (1992)]