Something along these lines, probably...
1. Have hope; things will get much better. In fact, things will get very much beter in a couple of years.
2. You're about to encounter Camus and Nietzsche. Read them both carefully, but take the first to heart, not the second.
3. Avoid becoming condescending or thinking of yourself as special/superior. You are clueless about how clueless you are.
4. Go easy on the alcohol and marijuana.
5. Follow your own lights. What other people think of you is really not that important.
6. There is a God. You have an immortal soul. It'll take a while to figure all that out, but you will. Don't buy into the atheism.
But then, I wonder if all of the mistakes are necessary. Would I be who I am now, had I not taken these "detours" or made these mistakes?