I read this book in middle school or before, which would mean publication in 2002 - 2003 at the latest, though I couldn't tell you how old the book was when I read it. I started writing down books I read in 2004, so that helps. I remember very little.
It was in the young adult/teen section of library, but it could have been mis-shelved. Setting is medieval-ish, probably UK/Europe, forests and towns.
Female main character is not quite human (I think), or maybe is very hideous. She lives apart from humans, maybe in a forest. One day she sees a human man and falls in love. He is already engaged to a woman (maybe). Main female shape-shifts or disguises herself to look like a normal woman (possibly Man's love interest). She wins his love, something bad happens to his fiancée (maybe main female did something?), and then her disguise slips/she shifts back to show her true self. He is horrified by her true form. She tries to make amends/is banished back to forest(?). She either ends the book lonely, transformed to an inanimate thing (stone?), or dead.
The tone/style is older, if that helps, like the tone of "A Wizard of Earthsea," or "Maven Many-Shaped."
I seem to remember stonework as setting being prominent. The cover may have had a female-looking woman-thing with inhumanly large eyes in a forest or in front of stone ruins. The cover art was also an older style.
I've been looking for this book for a decade now. I appreciate any help.
ETA 2017-02-05: Updated timeline, made a few small changes in phrasing. I am still looking for this book. The tone might be described as "gothic." As I got it from the library, I would think it unlikely that it was a completely unknown book unless it was by a local author (greater Rochester, NY area).
ETA 2018-06-10: Though I think it unlikely the author is completely unknown, I also think this was not one of the more well-known authors. I worked at this same library for a four years after reading this book, and I never saw it again though I kept my eyes peeled. (And the style didn't quite match any author I recognized.)