Being too ambitious in university and trying to do a three-year degree in two years, for essentially no real reason. Getting a burnout, recovering somewhat, and then trying to catch up by doing a lot of extra work each semester. Being able to sustain this for about three quarters of each semester, until I'd end up dropping several courses and accomplishing less than I would have if I'd taken a lighter course load from the start. Essentially repeating this until I'd finally finished my three-year-but-going-to-try-doing-it-in-two-years degree over a period of five years.