Living.
Something that most adults end up taking for granted from the first heavy workday. In fact, it's something that humans tend to take for granted sometime after puberty kicks in as responsibilities grow more and more plentiful and as aspiration and making of a livelihood become more and more of a perceived priority.
Bottom line: living is the one thing you have to always be mindful of doing as you become older and more successful and accomplished in your goals. Plain and simple.