I don’t remember the name or author. I had this book as a child in the late 90s-early 2000s but it could be older. It had really beautiful illustrations with a cut paper kind of vibe? It was about a spoiled, selfish girl, maybe a princess. She drained all the water from a fountain and killed all the fish. And there was some kind of ceremony to find her a husband but she rejected all the gifts and was rude and she was sent away. She found a unicorn in a thorn bush and rescued him even though she got pricked by the thorns and tore her dress to bandage the unicorn’s leg and then she learned her lesson about being a jerk.
The mention of cut-paper illustrations reminds me of Joan Aiken’s various stories illustrated by Jan Pienkowski. You can see examples here: https://joanaiken.wordpress.com/2019/05/15/the-wonderful-world-of-joan-aiken-jan-pienkowski/
I wonder if the tale you remember might have been one of the fairy tales from “The Kingdom Under the Sea”, although I can’t guarantee this as it’s years since I read it.
Hey I posted this ages ago but I found the answer because my parents still HAD THE BOOK!! It’s The Singing Ringing Tree by Louise Brierley