Have you ever felt like you don't belong? Or that you're not good enough? (Physics Undergrad)

I am a sophomore studying physics at my state university. I love my peers and my professors, but I am struggling with the work. I was a 4.0 student my past two semesters, passing through Calculus I,II, and the first couple physics classes. Now I'm doing significantly worse. I've failed both my past Calculus III and University Physics II exams. (May be partly caused by worse professors.) I'm starting to feel that maybe I am not smart enough for this program and my peers are better than I am. Current or past students of physics, did you ever feel like you weren't good enough for your program? Now that you have a career in the field, or have moved on to higher education, do you ever still feel this way? If so what can I do to improve my academic confidence.

Thanks,

-A struggling, yet hopeful physics undergrad.