Real discipline is not sticking to a practice schedule or spending xx minutes on scales and xx minutes on arpeggios every day. Real discipline is also not practicing a certain number of hours each day or playing a hard section 20 times straight.
Real discipline is knowing when to stop. It’s knowing when working on a bigger chunk of music is not helpful–real discipline is having the mental fortitude and focus to break a hard passage down into even smaller sections. Real discipline is only working on one of those sections until it’s perfect, resisting the urge go on and keep up the rhythmic flow. It’s listening to your body and knowing when your hands and head need a break.
Real discipline is also knowing what level of repertoire is appropriate for you and, for the most part, sticking to it. Sure, play some challenging pieces, but try to balance those with pieces well within your skill level.
Real discipline is hard.