First of all, you want to be a dentist I assume and the dental assistant position is just a side job, right? If that is the case, you can't really decide dentistry is not for you based on your work in something different. For example, you can't really work as a hygienist and then decide dentistry is not for you because they are two different things.
Second of all, assuming the comment on you, still one review should not break you, but rather encourage you to work harder and do better. This bad review should give you more determination to push yourself and do better.
Third of all, who said it's about you? It might be on some other assistant, don't assume something and feel guilty about it without making sure it's you.
Lastly, who said this patient is not just picky or whiny? Or over-dramatic or just mean? if you feel bad about every angry or dissatisfied customer, you will go nuts!
Last time I went to the dentist I felt mortified because of the dental assistant, I wanted to leave a bad review and I was so sad because of that woman, but I told myself, what if they fire her. I don't want to hurt someone like that. Also, the dentist was not nice to me as well. I don't know, I didn't want to leave a bad review.