Nothing can decimate confidence like negative thinking. Continuously thinking that you are going to mess up may actually cause you to do so. It is the ever-fearsome self-fulfilling prophecy. Likewise, putting pressure on yourself to do well may not yield those results, because your concentration will be focused on what will happen if you don't succeed, instead of doing your absolute best. Reciting the mantra "I can do this" over and over in your head can normalize the belief that you actually can do it and thus, may relieve any self-doubts. Remember to take deep breaths, believe in the strength that practicing gives you, and you can go on to do your best work.