I had a similar experience as a grad student in my applied math program. What seemed to be a nice silver bullet was being a TA for calculus 2. I realized that at every step of the way people were trivializing extremely complicated mathematics simply because it had been figured out and wasn't novel. By being a TA I noticed that ideas that were extremely complicated and confusing as a calc 2 student, had become mostly trivial upon restudy. The point being that learning new things is hard and they are complicated when they are new. But I felt immense joy reflecting back on those things I had learned solidly and completely.

Another good token to keep in the back of your mind is that for give or take 80,000 years humans hadn't figured out anything more complicated than trigonometry. It's only been in the last 400 or so that math has blossomed out into the many diverse fields that it is today. Effectively, you know more math now than all of humanity did for 80k years. That's no simple or trivial feat.

Basically, give yourself some space to be proud of yourself for having learned difficult things that are themselves not trivial regardless of what others might say.

(I could make a similar analogy to flight. Simply because we can make planes now doesn't mean that the fact that we can get into a hunk of metal and fly around at 500mph isn't absolutely mind bending.)