What’s the hardest part of discipline that nobody talks about?

For me, it isn’t starting. It’s the middle, the part where the goal stops feeling exciting and it is just repetition. That’s where I slip, not because I do not care, but because it gets boring and life gets loud. I’m curious what “the hard part” is for you. Is it staying consistent, dealing with setbacks, saying no to distractions, or believing it is worth it?