Once we realize that imperfect understanding is the human condition, there is no shame in being wrong, only in failing to correct our mistakes.
"The most rewarding part of acting is the discovery of the unknown within yourself."
I always try to stay true to myself and to play the game the right way.
So given a Time Turner, I would tell my 21-year-old self that personal happiness lies in knowing that life is not a check-list of acquisition or achievement. Your qualifications, your CV, are not your life, though you will meet many people of my age and older who confuse the two. Life is difficult, and complicated, and beyond anyone’s total control, and the humility to know that will enable you to survive its vicissitudes.