ok so i read this YA book (?) i think? back in middle or high school and it was probably already at least 5 years old by then, it was about this girl-monster who fell in love with a prince; she had bat wings and grayish skin and yellow eyes? i think. and could fly and ended up changing into the shape of his intended bride without him knowing it; she thought she was the last of her kind but it turned out that there was another who was like, the local wizard or something who lived at the castle; at the end of the book she ended up turning to stone because of a curse? or whatever deal she made to be able to shape-shift into the form of the princess, and she sort of died / froze with a locket that he gave her in her hand.
i’ve tried searching for this with keywords like girl-monster and shapeshifting, but all i’m finding is mortal instruments and terry pratchett stuff; it was a single book and not part of a series