My biggest regret is finding this website and wasting years of my life.
My second biggest regret is slowing way down at the 350m mark of an 800m to hit 400m in 60s when I saw a time in the 40s and thought I was out too fast and could save my race by conserving energy. No, all I did was cost myself a lot of time, as I could not speed back up much and ended up running 2:06 when I probably had a sub 2 in me with something like 55/30 (1:25)/34 (1:59). That year I had tendonitis, couldn't run more than a mile without my knee locking up, and all my workouts consisted of 45 minutes of pool running.