Your undergraduate physics education is a series of approximations. Every so often, you will go to a new class and the professor will say, "Okay, that was nice

Your undergraduate physics education is a series of approximations. Every so often, you will go to a new class and the professor will say, "Okay, that was nice, but that was an approximation. You were wading in the shallow end of the pool. Let's go a little deeper. Throw away what you thought you knew and how you previously modeled things, it isn't as accurate as this ..." and then you're on to the next thing. QM is one of those transitions and the jump bothers many.