I would define self discipline as the process of honoring a commitment you have made with yourself.
That means doing whatever you have committed to whether you feel like it or not.
This is a big deal, because it’s much easier to exert self discipline with outside commitments – for example, work. There you are obligated to honor your contracts, and there are consequences if you don’t.
It’s a lot easier to give up on yourself, because no one else is watching but you.
Maybe the ability to exert self discipline when no one else is watching is really the only discipline that matters.