Ooh, I like this one! I'd tell myself to be more confident. Looking back, I missed many opportunities in life just because I was too scared to try because of a lack of confidence. This has changed over the years and I can see how I would've responded much differently to situations, as I am today. 

You don't know if you'll fail, if you never give it a shot. Don't let fear of the unknown and low confidence get you down!!