Get Over Yourself

C.S. Lewis said, "True humility is not thinking of less of yourself, it's thinking of yourself less." 

This is especially true when singing. If you have your mind on yourself you will invariably become too self-conscious and tense, which will close up your throat. If you concentrate on conveying the story to your audience you will be thinking about how to bless and inspire them. That should keep you loose, open, and able to sing without fear.