Positive mindset (as long it isn't toxic positivity) can help you trough life, as it can help with your attitude making heavy things easier to bear. It can help you be more productive and also make you to become a person who other people find easy to be with. But that's about it.
It doesn't have anything specific to do with gambling, not to mention about results. It doesn't affect your winnings because positive mentality is not some magic.
This being said, it probably doesn't hurt you to have positively attitude when gambling if you aren't counting heavily on it affecting results.
True, having a positive mindset while gambling doesn't hurt and probably is the best way to gamble but sadly it doesn't contribute or add any significant value to your gambling activities. The outcomes would always remain the same whether you are positive or negative and positivity doesn't attract luck. Luck happens on it's own based on timing, being positive is good but it can't change the outcome of gambling