It’s easy to confuse plans and decisions. I can “decide” right now to go the gym to workout tonight, but that’s not really a decision. That’s a plan. If tonight rolls around and I decide I’m too tired to go – my earlier “decision” was meaningless.
I can come up with a plan for the future and I can commit to the plan, but when the time comes for action, I can decide to act in such a way that keeps my previous commitment (or not).
When you think of it like that, you realize you can’t really decide you’re going to do something in the future. All you can really decide at any given moment is – what am I going to do right now? (For me, that’s kind of a simplifying, liberating thought.)
Say that I spend time at noon “deciding” (actually planning and committing) that I’ll go to the gym that night at 7:00. Then 7 rolls around and I re-debate the issue. In that case, I wasn’t really committed to the plan. Discipline is making your current decisions/actions consistent with your previous plans/commitments.
That doesn’t mean that planning is useless. The point of planning (ahead of time) is to make better decisions (in the moment). If you plan and commit and tell yourself you’ve decided, but don’t have the discipline to follow through, you’re going through the motions, wasting time, and kidding yourself.