Book about a changeling child - found!

[soundofsunlight]

There are folk tales of fairies stealing human babies and leaving a changeling in their place, and in this book, that was exactly what happened. The changeling is the main character, so the story is from her point of view. She is a thin and sickly child, and she has an unhappy childhood because everyone is suspicious of her being a changeling (and they are correct). She does have one friend, though; I can't remember why he is friends with her when everyone else shuns her. He might be an orphan or something?

Anyway, since she's so unhappy in her life, she decides to run away and find the land of the fairies, and her loyal friend goes with her. They rescue the girl who was stolen to be a servant to the fairies, and take her home to her parents, and then...I think she and her friend continue traveling, since they don't really fit in anywhere, except with each other.

Based on other books I was reading at the time, and what I remember of the physical book, I think the book I'm looking for would have been published somewhere between the 1940s and 60s, maybe 70s.