I'm currently taking Fourier Analysis that uses Stein and Shakarchi Fourier Analysis textbook. I only have experience in multivariable calculus and linear algebra, and since they were the only prerequisites for the course, I decided to take the course.
However, the textbook is too difficult to understand, and I'm spending more than 15 hours/week to read a section of the chapter. Does anyone have any suggestions about which subjects I should study to get a better understanding of Fourier Analysis? I already read some basic textbook about Fourier Transform, and I'm confident with those calculations.