It's a natural emotion. When you first hear of the contest then of course you would feel confident in your ability. But when the time is nearing then you can feel your heart pounding like it's no tomorrow. You're nervous, it's understandable. Even actors and singers still get nervous despite their experience, yet they still manage to do well in the end. As much as anxiety is a nuisance do deal with, it's not impossible to control it so it doesn't affect you as much. Remember, your singing is your priority, it's the only thing you need your focus on.