the mistake 99% of traders make is thinking about money at all. money and P&L is an incredibly emotional subject. the objective in trading is to remain unemotional as only then can one make rational decisions. I spent many years in both the chicago pits and on the screen and there was a single ability that the handful of guys that were truly great possessed and that was the ability to take a loser and be unaffected emotionally. I can't tell you how many guys I knew including established traders that would have a loser and for whatever reason (money, ego) be unable emotionally to cut it loose. when that happens not only is one turning a manageable loser into a P&L disaster/potential blowup but also missing out on the opportunity of the next trade. mastering the skill of being unemotional took my trading to a completely different level. I'll leave you with this thought - THE BEST TRADES I HAVE EVER MADE AS A TRADER WERE LOSERS.