Listen to extremely successful people answer this question and you get pretty bored pretty quickly. “Take it easy, it’ll all be ok”, “don’t stress so much”, “love and live more”…
Their earlier selves were clearly serious strivers. It’s a nice thing about our world that the very successful got there by self sacrifice and hard work. But for some people, myself included, the advice to the younger self should amount to something more like:
Care more, relax less, life is short and you’re losing.
My younger self did didn’t need relaxation. I needed to correct the mismatch between ambition and effort. I needed pressure. Real pressure. I got there eventually but got there late.
Real pressure isn’t good for everyone so nice for me that it hasn’t driven me insane. And also good for us all that real pressure is easy to avoid in our society if it isn’t internally generated.*
But dodge that pressure at your ambition’s peril, if you care about that sort of thing.
*At least for the middle class, anyway. Certain segments of our society experience real pressure in living normal life (violence, poverty, addiciton, etc). That’s a different problem.