ok. i have the same problem... have someone あなた are comfortable 歌う in front of for example: friends,family, bf または gf. if there not ther just picture they are!!!!!
Always tell yourself that you're great! When あなた get another chance, know that it's not like the will yell that you're horrible. There's no need to be embarrased.
Close your eyes and feel the music. If あなた just sing the music, you're not going to be nearly as impressive. If あなた put your ハート, 心 and soul into the music, you'll forget about everything around you, and only think about the music.