I think I would just tell my younger self to start building connections with professors a little bit sooner and go to office hours. Shadow doctors in your free time. Try new clubs. Keep an open mind with everything.
I would probably tell myself to not lose the joy I have for learning and to make sure that I have better study habits in place and have more of a routine. Especially for a doctorate level program, mindset and routine is everything.
The advice I would probably tell myself is to take it a little more serious and just do the little things every day. It’s kind of like the snowball effect in a way, or compound interest. You do a little bit every day and it builds and builds and builds.