Act like the person you are, not the way they make you act.

[Cada uno ha de obrar como quien es, no como le obligan.]

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

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 165 (1647) [tr. Maurer (1992)]
    (Source)

On waging war honorably. (Source (Spanish)). Alternate translation:

All men ought to act according to what they themselves are, and not to what others are.
[Flesher ed. (1685)]

Every one must needs act as he is, not as others would make him to be.
[tr. Jacobs (1892)]

Each must act for what he is, and not for what he is held.
[tr. Fischer (1937)]