Know your major defect. Every talent is balanced by a fault, and if you give in to it, it will govern you like a tyrant.

[Conocer su defecto rey. Ninguno vive sin él, contrapeso de la prenda relevante; y si le favorece la inclinación, apodérase a lo tirano.]

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

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 225 (1647) [tr. Maurer (1992)]
    (Source)

(Source (Spanish)). Alternate translations:

To know ones prevailing fault. Every one hath one, that makes a counterpoise to his predominant perfection. And if it be backt by inclination, it rules like a Tyrant.
[Flesher ed. (1685)]

Know your chief fault. There lives none that has not in himself a counterbalance to his most conspicuous merit: if this be nourished by desire, it may grow to be a tyrant.
[tr. Jacobs (1892)]

Know your chief weakness. No one lives without some counterweight to even his greatest gift, which when petted, assumes tyranny.
[tr. Fischer (1937)]