I would tell my younger self - well - to think about anything other than yourself. In other words, worry about the present by understanding the past, while always entertaining high hopes for our common future. Maybe that's what I'd say. But I doubt it.
The longer you live and love, though, guys, the more real the question becomes: what wisdom have I really acquired in my life; what is it that I could have achieved. What true wisdom can I honestly pass on.
Uncomfortable stuff ;)