Wish I had of saved more money when I was younger...
Wish I had of stuck with my formal music lessons when I was a kid
Wish I had of bought gold... bought one gold coin and thought "may I should put half or a quarter of my savings into it"... my wife and I decided it wasn't worth the risk, and within 12 months the price had skyrocketed...
So many things, but you've got to look forward to things, if you live in past regrets a part of you will die there.. like Elsa said "Let it go"