This sounds like a very frustrating situation, I am sorry you are dealing with it. I hope you have luck in finding a new job soon, it's certainly no fun being stuck at a job you hate with a manager you feel like doesn't value you.

And there is truth to what hvert says, there is just something about being watched that can make people prone to making mistakes. I'm currently unemployed, but I've worked the majority of the time since I was a teenager, and I can attest that I might go all week without making one single mistake. Then Friday afternoon the head honcho comes in and starts watching me, all of the sudden I make mistakes left and right. I just do not like being watched, and it seriously affects my performance.