You can learn to have better willpower by exerting your will.
Many of us fall into the trap of waiting for motivation. It never comes. Not if we wait. Motivation comes if we stop waiting and start doing. It takes willpower to begin. You must constrain yourself to pick up your tools and get to work. Then motivation falls out of the sky and keeps you moving.
When you least feel like doing what you should, that’s when it’s most important that you cut against that grain. It develops willpower and discipline which makes getting started and getting that motivation easier next time.