In learning chess, I found that I was having massive swings in my rating--up to a 150 points at a time. Since I am not learning (or forgetting!) chess that fast, I've been forced to consider the psychological reasons for this.
Research has indicated that you are less intelligent while you are angry. It may seem smart to get 'all pumped up' before a game, but there is a reason when you watch GM's play that they aren't fist-pumping and shouting during matches.
Any and all emotional content is energy which you cannot apply to the mental realm of understanding and improving your position.
What I found is that when I was winning, the key is that I was relaxed. I didn't have a grudge. I wasn't letting my ego participate. I wasn't angry (either with myself nor with my opponent).
And I enjoyed the games a lot more.
What do you do psychologically to prepare for a match? What effects do emotions have for you?