Let me define the question better. For my purposes I'm saying a person "understand quantum mechanics" when they have what it takes to write a basic graduate level QM textbook. Maybe I'm setting the bar too high, but I'm a first-year physics grad student who can get good grades in QM classes but I don't *get* it, and reading through a book like Shankar or Sakurai or even Griffiths or McIntyre I see plenty of stuff I never would have thought of. I realize QM is the work of many dozens if not hundreds of geniuses working over decades to put it in its modern form, but I feel bad that I keep forgetting things I learned two chapters ago or that I don't know how to solve the end of chapter problems without help, etc. Sorry if this question is asked a lot, I couldn't find an equivalent to it using the search.