Considering I read/listen to 100+ books a year, and have been doing so, but to a lesser extent since I was nine, yes I most certainly forget. A LOT. My preferred genre in paranormal, so my brain can only keep track of x-number of werewolves / vampires / demons / witches at one time before the worlds start to overlap. I mostly rely on authors and the quasi-essential “recap” chapter to refresh my memory when a significant amount of time has elapsed between installments. Very rarely will I do a re-read (unless it’s a favourite author), but I will occasionally peek at the last pages of a previous novel to help fill in some of the blanks. I find that checking out my older reviews can be a huge help as well.
I think I work mostly off of situational memory, meaning I’ll often forget that I’ve already watched/read something until I can put it into context. For example: whenever I start reading a new book in a series I’m already familiar with, the previous plots come back to me because my brain gets triggered by the writing style, characters, would-building, audiobook narrator, etc. This is also the main reason why I have such an easy time coming up with similar titles whenever I get asked for recommendations by followers. Ask me “hey, I like books with love triangles, snarky heroines, that take place in a school setting,” and I can probably list off a handful of novels because you’ve just given me the context.
My guy on the other hand, can basically scan his memory for keywords and spit out an answer at the drop of a hat which is both a blessing and a curse because I’m always the one who comes off looking forgetful. He can repeat our conversations verbatim and never misses an important date, so whenever we have communication problems it’s usually my fault. LOL