If you're goal-oriented (and who isn't), major excitement, anxiety, nervousness and so on...all of them will negatively affect your performance when it counts most. Most of the really dependable bowhunters I've known were/are very cool machines at the moment of truth. However it happens, the ability to go into 'full predator' mode is important. I describe it as absolute total focus on the situation and approaching shot opportunity, while shutting out all emotion. I've often said that in the last 15-30 seconds I completely chill down. I need to be completely calm, and execute a clean, emotionless shot. There will be a time for emotions, but not when I need to kill.
I think one thing that really helps is being comfortable in the presence of big game animals. I suppose that comes from experience and maturity. I don't view a huge buck, bull or bruin as anything other than another animal...albeit one I may decide to kill. I found out a long time ago that some guys get really rattled by giant bucks or the sudden appearance of a nice bear right at their tree. The subsequent shot often goes somewhere other than intended, and disappointment results.
Murphy's Law as explained in math: The probability of an outcome is inversely proportional to its desirability. Think about that one.