Positive mentality is not a sort of magic that gives a gambler winning but it will help the gambler stay away from compulsive or unhealthy gambling practices. Besides, there are gamblers without this mentality yet they can be grinding better results than the gambler with a positive mentality. However, the difference and reaction lies at that point when they are not winning. Positive mentality will make the gambler understand that even if he losses today he can win tomorrow and so there is no need trying to chase loss knowing that if it is his day to win he don't have to stay in the casino for a long time. Sometimes, the first attempt can be a shocker and the gambler will win an unexpected amount but this feeling still won't make the gambler gamble uncontrollably. This is the positive mentality.
Absolutely right and I want to say that gambling, like betting, consists of two parts, the psychological and the monetary side and as you correctly noted, if a gambler or bettor has a positive attitude, then he will not fall into despondency and depression.
But then he is not insured against the fact that he will simply start losing money on incorrect bets on sports or in casinos. But this is also very important, and something needs to be done about this aspect too.