Discipline Fatigue is a Myth
I read an article a few years back describing how "willpower is somewhat like a muscle in that it can get fatigued if overused," in The New York Times. After reading that statement, unfortunately, I started using it as a mental excuse for every time I skipped a workout, ate a box of cookies, or forgo working on a project in favor of binging on a TV show.
Somehow, I let that one sentence influence how I viewed discipline and willpower. "Oh, I used up all my willpower by getting up at 5 AM all week, so I'm going to sleep until mid afternoon and veg out on the couch," I'd think.
Really, it was a convenient excuse for not having to do the hard stuff, like work on actual projects that'd advance me to long-term goals; instead, I had an easy out: Oh well, discipline is zapped for the day! Time to slack off.