Self discipline breeds self confidence. Self discipline builds trust in yourself. The more trust you have in yourself the more confident you are.
When your brain challenges you to do something and you actually do it, you deliver on your promise. When you tell yourself: ‘i’m going to do X’ and you don’t deliver, you hurt the trust you’ve built up over time.
You don’t have to worry about an internal struggle between different parts of your brain once the trust is there. You know you can trust yourself and that you will deliver.