Studying mathematics - I wanted to prove to myself that I could do it. But I already had doubts at the beginning of my studies: I understood next to nothing in the lectures and I couldn't do much with the problems on the exercise sheets. So I promptly rattled through the first exam.
Then I found two fellow students who motivated me to attend the lecture again with them and solve the exercise sheets together. That was the breakthrough for me. If one of us didn't get anywhere, someone else was sure to have a new idea that got us further. Once we had solved a problem, sometimes only after several hours, we were really proud. And if we couldn't come up with a solution after thinking about it for a while, there were always nice and competent tutors who were happy to help.
There's no question about it: the knowledge you acquire as a student teacher of mathematics during your studies goes far beyond what you learn in school. But this is the only way to get an impression of what mathematics really is, and it may give you the chance to give your students at least a small idea of what "real mathematics" is like. It also gave me the feeling that no student question could embarrass me so quickly, since my expertise is far greater than that of the high school graduates.
Studying mathematics taught me that it pays to persevere, and that two or three people often have better ideas and are more successful than one alone.