Request #211

Please help. This was read in the late 70s/early 80s (5th/6th/7th grade?). The main character is a overweight girl from a wealthy family accustomed to getting her way. A parent may pressure her to rake better care of herself. She feels unwanted/unloved(?), I think she hides in a large forest, pretending to runaway from home, and gets lost. I vaguely recall her hiding in a hollow tree for some time. She learns to survive (primitive survival) in the wilderness. When she is found much later, she has shaped up both physically and matured emotionally.