I'd go back in time at least twice. First, to talk to my 15 yo self and tell him to tell my class mate that I was in love with but I never got the courage to tell him. Then I'd go to my 24 yo self and tell him NOT to go on that excursion organized by the users of a radio forum. Cuz if I hadn't agreed to that excursion, I would have never met that beautiful heartbreaker snake who destroyed me emotionally and turned me into the cynic that I am now.
And finally, I'd go back to my 15 yo self to fuck myself.