"Obsessive-compulsive disorder sign", the more you stop thinking about it, the more your mind will force you to think about it. So let it be, and the more you try to stop yourself from getting nervous, the more nervous you will become. I was like this before, so I just ignored him and it felt better. I even sang in my head during the high school entrance exam. Once you really do the questions, you won't think about them anymore. The important thing is that you don’t think repeatedly in your mind whether to sing or not. I’m nervous again and there’s music in my mind again~~~