On the surface, it seems like a strange idea:  We become winners when we learn how to lose.  It’s when we can’t accept losing that we hold onto trades long after they’ve gone against us.  It’s when we can’t accept losing that we play small ball and never pursue meaningful rewards.  It’s when we can’t accept losing that we become frustrated during drawdowns and lose our focus and discipline.  It’s when we can accept losing that the good entry in a trade is never good enough and we miss opportunity.

So many of the challenges of trading psychology are rooted in our inability to lose.
When we can accept-truly accept-That trading is a probabilistic game, that losing is part of those probabilities, and that we can trade well and lose, our egos no longer become attached to our P/L.  If we go to a Vegas casino and bet large when we hold two pair, we can lose when someone around the table comes up with three Kings.  We were right to bet large and, over time, that bet is likely to pay off.  If we can’t accept losing, however, we will not just lose but be defeated.  That will prevent us from placing the next solid bet, or it could frustrate us and place us in revenge mode, betting even larger on a poor hand.
Learning how to lose means losing in a planned manner.  It means knowing when our trade has been proven wrong and exiting at a point that we have rehearsed and accepted.  It means sizing our bets so that the probabilities going against us won’t take us out of the game.  If we run a 60% win rate with our trades over time–which would be quite good–the reality is that we will lose several times in a row many times during the year if we trade often, simply by random chance.  If we can’t accept that kind of losing, can we truly hang in there to win?
Traders work on improving their trading and that is necessary for success.  Working on losing is equally important.  A great topic for a trading review is:  
Did I do a good job of losing?  The odds can never work in our favour if we don’t fully embrace those odds.