Does your boss do this to others? If so, you should all get together to talk to him about this. If not, then he is definitely targeting you. If it's just you, then you need to tell him nicely that you work better when people are not breathing down your neck, as this makes you nervous, and more prone to making mistakes. If that doesn't work, then talk to his immediate supervisor about this. Just tell them the same thing you told him.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau