The only thing I might tell my 16-year old self would be "I think that eventually no matter what you feel right now you will eventually be happy." No advice, no suggestions, just hope. Even that might be too much.
So anyway, I don't want the power to go back in time and talk to my young self. What I would like, however, would be the ability to do it for other people. Impersonate them and go back to talk to THEM when they were young. I don't want to fuck up my life, but theirs? Hell yeah. Like take that girl you had a crush on in 11th grade. Wouldn't you like to go back and tell a 16 year old her "Your greatest regret in life is going to be not routinely blowing Ben when you had the chance. This will haunt you for the rest of your days and you will die shedding tears over it." Okay, that might be a bit harsh, but there are others that you know YOU want to do.
"When you die and go to heaven you're going to find out that Jesus hates you. He fucking HATES you man. Hates YOU. Why? Because you're a douche, Billy. You're a douche bag. You go around blasting the gays and the Jews in Jesus' name, and he can't stand that shit. You know what? Jesus was gay. You find that out when you go to the pearly gates, see him making out with Peter, and then get sent down to hell. So think about that Billy. Pray on that motherfucker."
"Consider a career in lion taming. I really think we'd be happier if you went that way. We'd all be happier."
"Don't lose the weight."
"At some point in your life you will think up a character named Jar Jar Binks. At this point I want you to ball up your hand into a fist, hold it far from your head, and slam it into your jaw until you no longer have any desire to put that character into a Star Wars movie. Trust me, you'll thank me later."
"Never stop drinking. There may come a time when you want to, but don't. NEVER STOP DRINKING. Worse choice you could make. NEVER STOP DRINKING."