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.
I believe the point of discipline is to do something regardless whether you want to do it or not.
My favorite quote on discipline:
“A modern stoic knows that the surest way to discipline passion is to discipline time: decide what you want or ought to do during the day, then always do it at exactly the same moment every day, and passion will give you no trouble.” - W.H. Auden
Connecting the future outcome you want with your present experience. Running when it’s 40 and rainy sucks to start, but after your warmed up and for the rest of the day you will be glad to have done so. Starting definitely sucks, but if you’re convinced enough in the present that the future state is worth it, you will want to run and you will want to push through it. That’s a skill that can be cultivated.
But it’s also about discipline, this sort of thing can only be accomplished with discipline, which is what really lets you accomplish things.