Your Dentist Can't "Fire" You
Moving your teeth and the changes (hopefully good) to your smile are a big deal and it is perfectly normal that you might be feeling nervous about the process. People change their mind all the time, so I am sure your dentist will understand.
You can stop treatment at any time, but before deciding whether or not to continue I'd speak with your dentist about your concerns and maybe they can put you more at ease. Keep in mind that your dentist has already paid significant lab fees...