Main myth I regularly hear about gambling is how you feel effects your bets. Which I think is %100 wrong. Gambling isn't something sentimental, you maximize your chances by your own skill, risk assessment, your intelligence etc. And luck side of game is nothing emotional. Its just about possibilities. You try to hit %1 to %10 possibility. People should be more materialistic about it.
Well, that actually is more on Luck.
Gambling not in a good mood may not affect your bet but will affect the way you place your bets.
When you gamble with an upset mood and losses your composure, you might carelessly place your bets out of frustration and will drain your pocket in no time.
I personally experience that before when I had a pretty bad losing streak. Out of my frustrations, I've placed everything I've got left in my pocket to try to win it and make it even.
So, no it's not actually a hundred percent wrong. Bad moods will change the way you place your bets.