That little voice
Actors: In most of our brains is a little voice that serves a couple of purposes. Its main aim is to aid us in stage business: "Pick up the spoon on beat 4," "Your cue line is 'but I don't love him," "Left, 2, kick kick, step turn brush." Its secondary purpose is to second guess every move we make: "You forgot the spoon," "Don't forget the spoon," "You always forget the spoon," "You're a hack," "You'll never work again."
The good news: That helpful little voice never goes away.
The bad news: That neurotic little voice never goes away.
The better news: Since that little voice is YOUR brain, you have a means by which to deal with it.
A word about the helpful voice--it is useful to have this voice on the front burner in rehearsal. It keeps us on task, and helps us stay ahead of the action on stage. It serves continuity and pace. But we need to steadily transition that voice to the back burner so our performance isn't stilted and mechanical; doing all the right steps, but focused on doing all the right steps. At some point, this very deliberate behavior must bridge the gap between your intent, and the character's reflex. That way, "Pick up the letter opener, dammit" becomes "Nigel absent-mindedly picks up the letter opener."
A word about the less helpful voice--it isn't always useful. It keeps us on task, but from a pretty negative angle. It fills our heads with a lot of don'ts, can'ts, and nevers. Not especially beneficial to the actor's efforts--which are rooted in pursuing positive goals. If we can, we need to commit that little voice to the back burner permanently. AND--because it's all your brain--train it. When it mutters in your subconscious mind, "That note's too high. I'm not going to hit it," just add the words 'now' and 'yet'. "That note's too high now. I'm not going to hit it, yet." It leaves open the option for, "but I'm gonna." My less helpful voice was notorious for muttering "of course" anytime something bad happened to me. With a lot of wrestling, I banished it to the back burner, and replaced "of course" with "unfortunate" and helped retrain the voice to acknowledge the unpleasant nature of a situation without re-enforcing the belief in its inevitability.
The challenge with these two voices is they are one and the same. Sometimes it's of great use to the task at hand, and sometimes it seems there just to sabotage the good work you're doing. But remember it's YOUR voice. It's YOUR mind creating the text of the play being voiced. Write a play you'd rather hear. Bring it to the fore when it helps, and send it packing when it's creating mischief.
This is the result of a creative mind. It creates all the beautiful things you've created so far, but it also creates the weird little doubts and strange little scenarios that sidetrack us. Note it. Use it. Train it. It's a tool if you decide it is.