Yea it can be tough but its best to try and not get in your own head about it (easier said than done I know). As soon as you start thinking about messing up it makes it more likely that it will occur, you psych yourself out. Sometimes I will pause before a big fight, take some deep breaths, remind yourself verbally (out loud) to be calm, take it slow, be patient, speak your general strategy out loud to yourself. Then when you resume focus on the game and your gameplan, nothing else.