If you aren’t ready in here, you aren’t ready out there.
One of the reasons it’s a good idea to practice performing in front of your peers is because it’s a first test of your preparation. If you can’t sing it in rehearsal, confidently, from memory, with attention to all the details as you intend them, you aren’t ready to do it in front of an audience on stage.
Learning where the line is for “ready in here”, and making sure you are well over it, is an important part of becoming proficient at performing.