I finished reading that book, the one that I stayed up all night reading in my youth and then hated.
Looking back at it (coughcough) years later, I think my issue with it was that the ending was ambiguous - what did the antagonist/love interest do to the protagonist, if anything? What do the flashbacks mean? Is she really dead?
As a thirty-something I can read between the lines and come to a satisfying conclusion, but at the time I was so frustrated that it was ambiguous, and when I tried to talk to my mother about it the next day (well... eight hours later in the afternoon when I woke up, it was after dawn when I finished the book), she just laughed at me.
Oddly, while I mostly forgot about this book for years (and when I did think about it, couldn't tell you much about it), reading it again I discovered that both the plot and the format are quite similar to a novel I started writing a couple of years later. I don't think this is coincidence (I've read literally thousands of books, and very few of them made enough of an impression to haunt me a decade-plus later), but it also wasn't deliberate on my part.