What would you say......?

My 15 yr. old grand daughter asked this of me........

"..... i was thinking, if i could give advice to the twelve year old me i would have loved to. but i cant, its impossible so i was wondering, what advice would you have given your 15-16 year old self?
if you could come back in time to leave a letter, what would it say? honestly?"

Interesting question..... :)

I'm wondering ...what would you say? Even if it's just one line....I'm really interested in what you would say to your 15 yr. old self.