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.
I tend to agree with what @Oasisman said because the mood will affect a person in thinking and acting.
If they had a bad heart, as he said, it could have a bad effect and if it was a gamble, he could lose in an instant.
We've all experienced frustration, but many of us can get over it and get out of it for now.
But we do hear a lot of myths out there about gambling and maybe some work.