Bhagavad Gita, Chapter 18, Verse 33

The fortitude by which the functions of the mind, the Prâna and the senses, O Pârtha, are regulated, that fortitude, unswerving through Yoga, is Sâttvika.

Real, sustained willpower is described here as something built through actual practice, not a fixed personality trait.

Krishna ties this determination directly to Yog, a real, developed discipline, rather than treating it as something someone either has or doesn't. Real effort produces this steadiness over time.