what would you tell yourself?

[imaz]

Don't get married. Once is bad enough but three times is just willfully stupid!

[Nicky]

I would say
"don't write this letter, I have already read it"   

I would not really be the same person if I had done things different. Perhaps I would only write "things will work out ok, see you when you get here".

[Janet_Girl]

The perfect song of this thread. 

Brad Paisley - Letter To Me