Positive thinking gives you an edge every time a person is gambling. Base on my experience, every time that you have doubts on your card, no matter how strong it is, you will end up losing the game because you are not confident. Especially in the game of poker, the other player will definitely feel that you are bluffing if you're not confident. You are also willing to take the risk if you are thinking positive, the more you try, the more chances of winning.
Positive thinking in gambling does not give you an edge but throws you in a deep shit. If you want to have positive thinking, have it in a real life investment where you can count on your skills, your idea of the market to play your cards out right and with a little bit of luck.
Gambling however is all about luck and you really have no part to play as a person except gambling your pockets to the lower side, and with that, you will only end up marring yourself as a result of your positive thinking still it is theoretical. Personally I do not find any differences in gambling results so far by being positive in my experiences.