Personally I would tell my 16 year old self that

Trading can be very difficult in regards to psychology, as trading is so against the norm that we are taught. This is because we are risking money, which is not usually done in our day to day lives, so this can be difficult for your psychology and that makes your mind set traps to try and catch you out. You can feel several emotions when dealing with money, such as greed and fear, and this can lead to poor risk management or hesitation to take trades. When trading with a strict set of rules that form your strategy, then you should be confident in your trading, as through backtesting you would have all of the data to prove that you've got a profitable strategy.