I'd probably tell myself not to party so much or so hard and to focus on finding good friends. Just because you can depend on someone to get fucked up with you doesn't mean that you can depend on them. Building good relationships is so, so important and I wish I'd realized that sooner.
All the wasted money. The knife scar on my arm. My future. You hit the nail on the head. My 18 year old self wouldn't even recognize me. We certainly wouldn't have been friends.
Well, making lots of mistakes means lots of opportunities to learn. I don't know your situation, but hopefully your experiences will allow you to figure out how to go about creating the kind of future you'd like for yourself. That's what I hope for myself too.
Oh most definitely. Sure I'm not perfect but I wouldn't be who I am today without past challenges. Regret is a wasted emotion. I guess I just wish someone had told me (especially in high school) that the party doesn't last forever. Eventually you've got to sober up. Things are better now but my early twenties were one hell of a hangover. In an odd way I actually relish the hardships. I've met many people my age who seem completely oblivious to tragedy and I'm afraid for the hard fall they've yet to experience. Life's richer when you can look to your past and remember your rock bottom, and how far you've risen above it.