Practice Self Discipline

Self-discipline is a learned behavior.

It requires practice and repetition in your day-to-day life. Practicing self-discipline is like trying to build muscle at the gym, the more you do, the better you become at it.

The difference between an amateur and a professional is that a professional does what needs to be done despite how they feel. It’s about not letting your emotions dictate your actions. 
There are simple ways to practice self-discipline every day, the key is to set goals, plans and constraints for yourself; and stick to them even when you don’t feel like it.