Quote from illiquid:
Play poker or blackjack to train yourself to avoid results-oriented thinking on the most basic level.
If you double down on your 11 vs a dealer's 5 and lose, will you refrain from doing so again the next time around (given no extreme negative count)? Or worse, feel so indignant that you triple your next bet to get the loss back? If you're dealt 7 2 offsuit and fold, and the flop comes 7 7 2, will that make you think twice about playing those cards next time they come by? etc.
The market is no plain deck of cards and is much more difficult to place into that frame of consideration, but if you apply the same kind of thinking at least to your own interaction with the markets, you will have a much healthier attitude towards trading overall -- no room for fear or regret.