Taking a driving test can be a nerve-wracking experience, particularly if you have not been able to accomplish it before.
I suppose there are two suggestions that might help. The first is that many driving schools in some states actually use the same instructor as the examiner for the practical test. This makes things easier, it is almost as if you going out for a bit more instruction, you are used to the person and the car - as well as the area where you will be doing the driving.
The second is practice. I used to have to give lectures and tutorials before classes, and I found it was very easy for me to stumble and get lost if I went into them just having written notes and left it at that. Practicing delivering the talk made a lot of difference, at times even if one was unsure the habit one had acquired in the practice runs kept one going wihtout making mistakes.
So I guess you find out from your own knowledge or that of an instructor what are the areas you are weak in and practice them till you can do them wihtout thinking.