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.