Super Bowl 50 showed that nerves can get the best of anyone, including the league MVP, Cam Newton, and his offensive line. Unfortunately, most of us have experienced the frustration of being able to perform during practice, but "choking" when it's game time.
Feeling butterflies isn't inherently a bad thing. The key is to be able to get them to fly in formation. To do this, you need to control your emotional thermostat. It's important to remember that some stress is good. In fact, emotions motivate your competitive drive. But in excess, anxiety / stress impair performance. Psychologists suggest that meditation and guided imagery can help overcome these obstacles. By role playing situations, we're able to practice how we'll react, so we can keep our emotions in check.
But, obviously there are limits to this. Often there's no replacement for experience.