It has happened to every professional performer at least once. You're out there singing, it's wonderful, and then your mind goes blank and you can't recall the rest of the words. Or you have a closet malfunction--your trousers zipper is open or the hem of your dress gets damaged. Suddenly, rather of the performance of your dreams, it's a nightmare!
Perhaps you're a brand new performer and that hasn't happened to you yet, but you've seen it happen to others, even the best, and you're frightened it will happen to you. Or you're fearful the audience won't like you.
Have yourself in good health. Eat a nutritious diet and get plenty of sleep in the days leading up to a big performance. Drink lots of water to keep yourself well hydrated. Avoid negative thoughts such as "what if they don't like me?" Focus on the fact that you love to sing and you love the matter that you're performing.
On performance day, indulge yourself a little bit if you can. When you show up at the venue, check that the stage and all equipment are put together the way they're going to be.
Immediately before you perform, as you're waiting in the wings or the green room, have a few moments to focus on your entrance and set your mood. Relax, take a deep breath, and tell yourself that you've worked hard to make for this, you've eagerly predicted it, and that you can do it.
When you get on the stage, bow slightly to comprehend welcoming applause. As your accompanist takes her place, look downward and focus on the start of your song. As she begins the introduction, slowly raise your gaze to horizontal, as if you are watching a carpet lay open from your feet to the back of the hall.
As you sing, envision the meaning of the song to help the audience get into it. What you "see", they will see. Sing the song just as you have sung it hundreds of times before in practice. You know you can do it.
In the unusual event that something does go wrong, try to stay calm. If it's a simple issue like mixing up the words or singing verses in the wrong order, it's very possible that nobody but you and your accompanist will know the distinction. Keep going as if nothing happened. As for wardrobe malfunctions, unless it's a intemperate one the chances are that few people will notice.
After the performance, breathe a sigh of settlement and pat yourself on the back. You did it! Even if everything didn't go exactly as planned, you handled it with grace and fulfillment. Enjoy your well-earned applause.
If you weren't content with your performance, immediately afterward is not the time to dwell on it. Discuss it with your teacher at your subsequent lesson and learn what you can do to improve for next time.
With time and experience you will get better and become more comfy with performing.