I'm searching for a fairy tale, or a fairy tale retelling, that I've read in English. The story is about a Princess who turns into a monster or a beast at night and so is locked away. If a man can stay in the room with her for three nights without showing fear it will break her curse and he will receive her hand in marriage.
It goes something along the lines of: an ordinary man (not a prince) stays, slowly winning her trust and getting closer each night, but he realises that the situation isn't fair to her, so in the end he waits until the king comes to see for himself on the third night how 'the monster' trusts him, and he choses to show fear, so that she lashes out against the king and sets herself free.