The lesson is to have a pre-bet calculated plan. Where you only do certain amount of bets in a session and stop on designated target whether you lose or win. There is no emotion involve - only follow your pre-planned bets.
Stopping on target won't make you win in long run, also emotions won't help either to win in long run. You can control emotions and you can be cold as ice and it still won't change the fact that most of players will lose.
Or do you think this plan is working and everyone is winning and all gambling sites will close their doors? I don't think so.