My Ph.D. taught me that
Smarts and IQ don’t matter as much as everyone thinks. Can you solve a differential equation in your head? You must be really smart. But can you persevere when you cannot solve it? Can you go through 3 books and 20 journal papers to figure out the solution? That is what counts.
Murphy’s law is true. Be optimistic in general but prepare for the worst. Your code will not work, the results will not match even after multiple attempts, your papers will get rejected, your advisor might change your project after you have spent several months on it, you may have financial or health troubles etc. Can you, in all this chaos, remain somewhat calm and get stuff done? That is what counts.
Self-care is cool. Can you take care of yourself? Can you take care of your physical and mental health, force yourself to exercise regularly, eat healthy food and have a social life out of the lab? When you are starting your Ph.D., these things may seem trivial. But, believe me, you will burn out and given the prevalence of mental health issues in academia, it will get to you.
The value of a Ph.D. comes from the learning. Your Ph.D. degree is not worth much if you did not learn the trappings of your field AND scientific publishing. Throughout your Ph.D., you should be learning new things.