I'm the reverse: I booted up IN yesterday after a few weeks' hiatus, and the entire time I'm doing a Hard playthrough, I was thinking to myself, "what happened to all of the Hard Mode bullsh*t I've been putting up with?"
For me, IN was roughly linear in terms of difficulty increase between levels - the complexity, density, and speed of the bullets and enemies increases roughly the same amount between Easy and Normal as it does between Normal and Hard. This is important for me, as I only do general memorization; I derive my solutions to nearly all spellcards (and situations in general) in IN on the fly.
(That being said, Lunatic is a huge jump, and really breaks the pattern.)
PCB, MoF, and EoSD (as well as what I've seen of SA) are different: their difficulty curves are quadratic. Between Easy and Normal, all aspects I listed above increase between twofold and threefold.... and then do the same again between Normal and Hard, resulting in a much, much greater overall increase. I still do on-the-fly solutions (which is probably why I'm having trouble actually finishing the 1cc of PCB Hard, although I've come within a few seconds of doing so several times now), but they're much rougher, and the game is a real workout.
Some of our differences might just be in how we dissect the danmaku - point of view (or the ability to change PoV on the go) is critical, I've found. As is the way in which our minds perceive the board, and the avenues we're trained (through life in general) to go down as a matter of instinct.
Choice of character may also be a deciding factor. Who do you use?
