This is the part of you that says things like:
- I'm flat on that note... no, I'm sharp...
- I'm not feeling it right now
- I got this track in the wrong key
- I can't hit that (high) (low) (long) (soft) note coming up
- I should know better than to think I could do this
- I am not getting this fast enough
- I suck
- They (the people in the control room) think I suck
Now, I ask you: What part of any of those questions has to do with delivering a message? We can take a lesson from children and puppies who are much better at being present in the moment when they have something they want to "say".
The vocal booth has room for two people: the singer and the one (usually not physically present) being sung to. The time to critique is before you get into the booth (do pre-production to make sure it's a song or story you believe you can authentically deliver, and that the key is right) and after you leave the booth (while you're listening to playback with your vocal producer).
Singing, like writing, is a creative process. It's a simple mindset where you focus on telling the story with confidence and freedom. If you try to analyze, edit and critique your effort while you are in the middle of it, you will cripple your ability. Think of a child coloring a picture while a parent tells them what they are doing wrong and how doing something different could make it better. The kid's gonna throw some crayons before long and give up. Let the child color; discuss coloring techniques later.
Don't sabotage yourself. It happens all the time. Instead... choose to totally commit to your vocal performance. Talk to your imaginary friend- to whom the song should be directed. This person is rarely anyone in the control room. When you go into the vocal booth to perform, bring your inner creative child with you and leave your inner critic to cool it's heels till playback! You'll be so much better, I promise.