Ammani (http://jikku.blogspot.com/) is a writer I admire so very much. She had asked her readers to write a letter to their own teenaged self for "If I knew then what I know now".
This is what I would tell my own self: You never regret the things you did, but will always regret things you did not do.
It is here in Ammani's blogspot too: http://lettersforall.blogspot.com/
:-) Thanks for the opportunity Ammani....keep rocking!