We rarely practice under actual performance conditions. As a result, when the big day arrives and we are expected to perform, we fail because we've established a comfort zone that doesn't match the reality of a high-stakes pressure scenario.
We lose both focus and confidence because we don't have a consistent pre-performance routine that keeps us insulated from environmental distractions.
We worry about the outcome and repeating past mistakes for fear of being embarrassed and looking foolish. That leads to anxiety and over-arousal. Adrenaline surges, our heart rate soars, and, as a result, our cognitive functions are severely impaired.
We fail to recognize and take control of our internal communication, body posture, mental images, and emotions. As a result, we self-sabotage through auto-neurotic thinking and self-defeating behavior.
We fail to let go and trust our training. Instead, we over try, which blocks our instinctive skills developed during practice. Our conscious mind overthinks and gets in the way of our subconscious.

There is always a difference between your practice performance and actual competition performance.

Practice entails far less perceived pressure than actual competition. In this way, we must figure out ways to close that gap.

Many of us begin to think negatively in a competition, high-pressure performance situation, so do a mind audit prior to performing.

Source: Mindset Secrets for Winning by Mark Minervini