Discipline is the trait of doing what we have decided to do, even when the mood is gone. People often picture it as strictness, but it works best as a pattern of small, steady choices. Those who have it are not necessarily harder on themselves. They have usually built routines that make good choices easier. The Gita describes discipline in this practical way.
Gita 6.16 and 6.17 say that yoga cannot be perfected through extremes, and that a balanced way of eating, recreation, work, sleep and wakefulness creates the ideal condition for it, so build a sustainable routine instead of an irregular or extreme one. Gita 6.10 recommends practising regularly in solitude with a disciplined mind and a simple life, so create a quiet space and a consistent daily routine. Gita 2.50 calls yoga skill in action, aiming at excellence and clarity in work.
The Gita also points to regularity and attention. Gita 5.27 begins meditation with withdrawing attention from external objects, regulating the breath and directing awareness inward, and Gita 4.29 treats conscious breathing as a form of discipline. Gita 6.15 says that consistency, not intensity, gradually brings lasting peace. Gita 8.6 says that what one constantly dwells on shapes who one becomes, so choose carefully what you repeatedly think about. Begin with a small practice you can keep up.