It’s a children’s book from around the 1990’s. The main heroine travels to cure the curse of the Beast Prince. I think he had the head of a pig/boar. The plot is similar to East of the Sun, West of the Moon but that is not the book. Near the end the heroine must unlock a door but lost the key. She must cut off her finger and use it as the key instead.
The illustrations of the pictures in the book were soft and water color-like similar to Tomie DePaola I think. The heroine went on a quest and I think there was a glass mountain she had to climb?
The illustrations of the pictures in the book were soft and watercolor-like. When the heroine was on the quest I think she had to climb a glass mountain and at the top was the locked door. The heroine and the prince had a baby boy at the end of the story.