Enchiridion 37

If you undertake a rôle which is beyond your powers, you both disgrace yourself in that one, and at the same time neglect the rôle which you might have filled with success.

Epictetus, Enchiridion 37, translated by W. A. Oldfather

If you have assumed a character above your strength, you have both acted in this matter in an unbecoming way, and you have neglected that which you might have fulfilled.

Epictetus, Enchiridion 37, translated by George Long

If you try to act a part beyond your powers, you not only disgrace yourself in it, but you neglect the part which you could have filled with success.

Epictetus, Enchiridion 37, translated by P. E. Matheson