One of the big aha moments which clicked for me only fairly recently was staring at some physics equations my own internalized realization that light doesn't experience time and then I had to tell everyone haha - but that one eureka moment unlocked a whole lot of understanding and certainly a lot more questions. This of course was after all the schooling and physics where it somehow sailed over my head the whole time…
I was going to put this as mine. It’s astounding how important this is. I woke up at 2am with the intuition that light was like a lightning strike across spacetime, with no time and no distance between the emitter and absorber. Why and how, though. What are the fully-baked implications.
What I still don't understand, though, is how photons can "wave" or how "long" a photon is (or how they can have any "length" at all).
What I also didn't understand back then was how light can be slower when there is matter present -- or how a photon could somehow know that it was supposed to cause another, very similar, photon to be released in just the right angle when it hit something that, if zoomed out enough, looked like a mirror. How on Earth could the photon know that the photon it was hitting was part of a flat surface and what its orientation was? I even started using that mirror question as a litmus test of physics students once I started at uni (comp.sci.) -- pretty much all of them failed, not by not knowing the answer but by not understanding that this was even a question that required an answer.
I now know that the photon -- the wave(s) in the electromagnetic field -- cause atoms (and in particular their electrons) to move about a bit, which in turn cause waves in the electromagnetic field. And by adding up all these waves, we get the resulting wave which moves slower than c and which might seem to have been reflected at an angle. Why my physics teacher in high-school didn't tell us that, I don't know.