I'd tell myself 'you don't know anything, so stop acting like you do'. It's a funny thing, but I find the less you know the more you think you know, and then when you do slowly start acquiring knowledge you realise you don't really know anything. It's one of life's little paradoxes.
I'd tell myself 'be kinder to those close to you'. I suffered from 'centre of the world'
syndrome as an adolescent, was pretty selfish and self-absorbed and, truth be told, still am, be it to a lesser degree.
Finally, and most importantly, I'd tell myself to smile more and just enjoy life. I was too serious when I was younger, too concerned with being cool and all that rubbish. It might make you look like something in the eyes of others, it might get you laid, but the price for conforming is everyone likes you except yourself.