A question I answered on Quora: How do you keep yourself disciplined?
You don’t need to “learn” discipline. You already have it.
Think of the things you do consistently or do well or that you enjoy and look forward to (or things that check all three boxes): There’s Discipline there!
Maybe these are destructive habits, like eating bad food, smoking or procrastinating, but they are indeed disciplines. So I’d guess what you’re asking is how to learn discipline for doing things that you feel would help you, but you haven’t yet done.