I graduated from college with a physics degree faster than my peers (because of various reasons: financial constraint and ambitions as well as near-burn-out state). My undergrad is a decent research university, not the best of the best, but many well-known researchers. I did well in most classes but I sometimes think it's because my classes are not rigorous enough. Just saying my thought is not out of thin air because I attempted to take a graduate course at the time and I had to work really really hard for it. I don't think I really understood many things from undergraduate classes, even though I'm really good at solving problems from examples. Now I think many of the basic things I didn't understand before start to haunt me in my graduate program. Just to emphasize, the gaps I'm talking about is not just some details but in some occasions the whole subject. I still have many gaps from my peers at graduate school. Even though I'm finally filling in the gaps starting grad school, I still feel like there are a lot I need to revisit.
Perhaps my state is a result of me graduating early and pushing my schedule to a limit where I can barely sit down with a class and understand it. Perhaps I fooled everyone in grad admission into thinking I'm some sort of smart student. Perhaps it's because my undergrad didn't really have the best structure preparing me for what I'm doing now. Or perhaps I didn't really know how to learn effectively before graduate school. I don't know.
I feel worrisome sometimes about my abilities, but relieved other time when I revisit what I didn't know and finally understand it. I want to ask if other people experience similar things about not understanding many things and eventually filling in the gaps later (I mean a lot of gaps :)? Or is it a debt of a student who didn't properly go through their education?
As a final word, I know about imposter syndrome. I think I have evidence in my lack of qualifications as said rather than only disregarding my achievements. And thank you in advance for reading my long concerns.