Being well prepared goes a long way towards reducing jitters. We all get those feelings to some degree, but if you've done your homework, part of which should be visualizing yourself correctly carrying out the required procedures, you'll be fine. I, like you, feel the nerves up until getting in the seat and then I'm good to go. If you didn't feel some of this, you wouldn't me motivated to prepare properly, so it has some value. Imagine feeling irrationally cool to the point that you didn't do the work, and then looking foolish. There's a thing to fear.