Could he be right?

Hello, I am a prenursing student right now that is applying for nursing school. I had to take a CPR class today, and I have to add that I knew nothing about CPR previously. So he taught ( sounded like he mumbled the whole time) and we watched a video. When it was time to perform the CPR on a dummy, I did well in the chest compression part but when it came to the rescue breathing, it took me three times to get it right. I felt like he was looking for any chance he had to correct me, even when I was doing the chest compression he kept telling me to change my pace. Now here's the problem, when I didn't get the rescue breathing right the first two times, he said I better rethink wanting to be a nurse. I know it sounds stupid but that has really been bothering me especially since I'm the kind of person that has to practice something and make practice mistakes before I perfect it. He's a RN but teaches these classes, very blunt and has years of experience in the ER. I know for sure I want to be a nurse. Am I over thinking this? If I have to practice a lot does that mean I can't be a nurse? Has anything like this every happened to you? like discouraging you.