For the benefit of my past self: discipline isn’t about punishment or being harsh with yourself. Discipline is the practice of saying to yourself “this is hard. This is scary. This is worth it and so I’m going to do it because I’m stronger than I think” and then following through with action.

EDIT: Sometimes the "following through with action" bit also involves knowing what you need to set yourself up for success (e.g. good food, enough sleep, encouragement from friends, the ear of a therapist, guidance from a coach, clarity on a goal) and enduring the discomfort that comes from the decisions required to get those.