I was in a similiar situation when I was 17. My driving instructor used to shout a lot and make me very nervous and anxious. Luckily, my driving instructor was not self-employed, so after a while i took courage and talked to his boss about it and in the end i got another driving instructor (said boss).

In that time i did not realize the following: You are the customer of your driving instructor. So if you are not happy, it is your full right and you should not feel bad about saying that the provided service (the car lessons) are not adequat to your standard and you will seek for another instructor.

I would say something like

I think this is not working out. Thank you for your work but I will try the test with another driving instructor. How can we proceed we with the cancellation? are there any fees?

I was in a similar situation when I was 17. My driving instructor used to shout a lot and make me very nervous and anxious. 
Luckily, my driving instructor was not self-employed, so after a while I gathered my courage and talked to his boss about it. In the end, I got a different driving instructor (who happened to be that said boss).

During that time, I hadn't yet realized this: 
You are the customer of your driving instructor. 
So if you are not happy, it is your full right to change instructors and you should not feel bad about saying that the provided service (the car lessons) are not adequate by your standard and that you will be seeking another instructor.

I would say something like

I don't think that this is working out. Thank you for your work, but I will try the test with a different driving instructor. How can we proceed with the cancellation? Are there any fees?

By using the statement "You are the customer", I tried to refer more to the fact that you as a student are neither legally nor socially obligated to stay with the same driving instructor until passing the test.