2004, early summer, when I was 16: "You've ADHD and ASD, you should ask your parents to go to a psychiatrist, and everything will be better afterwards."
I'd tell me in high school to get treatment for depression instead of white knuckling my way through life and ending up with treatment resistant depression at 28 because I went without for so long.
I am much happier in my 50s than I ever was before- in fact, I was despondent for the vast majority of my adulthood. So I'd go back to pretty much any age before I turned 45, give myself the biggest hug, and say "It's going to be okay, you're going to make it, and you really are good enough."