I'm in my mid-thirties myself, so I am not exactly the wizened codger whose advice the OP was clamoring for. Even so, I'll approach this question as such: what advice would I have wished to have had been given in my 20s? There are three bits of advice that spring to my mind that, though I know them well now, I was only vaguely cognizant of when I was a young know-it-all in college.
1. Seek not to either stifle your emotions or to wallow in them, nor to elevate rationality above emotion or vice-versa. The goal should not be the elimination of desire and feeling, but rather its domestication. In the same vein, rationality divorced of feeling is both cold and deadening. Try to discipline yourself to think immediately as you feel, to question the impressions at the forefront of your mind.
2. Any person who can make you angry has made you a slave. If you cannot control your feelings, your tongue, or passions, then someone will eventually do it for you, and likely not to your advantage.
3. Read, read, read. And not just for pleasure, but with a critical eye. Stories let us playfully explore our morality and conduct in a controlled environment; histories let us know the ground on which we walk is paved with the dead, and that they whisper to us their hard-earned knowledge if we but listen; poetry nourishes the transcendent within us; rhetoric sharpens our minds and empirical studies provide us information about the world around us, a world which we may not always have the fortune to travel fully for our own sakes. READ.
