The friendship lie / by Rebecca Donnelly.

Enhanced descriptions from Syndetics:

Cora Davis's life is garbage. Literally. Her professor parents study what happens to trash after it gets thrown away, and Cora knows exactly how it feels--to be thrown away. Between her mom and dad separating and a fallout with her best friend, fifth grade for Cora has been a year of feeling like being tossed into the dumpster. But Cora has learned a couple of things from her parents' trash-tracking studies: Things don't always go where they're supposed to, and sometimes the things you thought you got rid of come back. And occasionally, one person's trash is another's treasure, which Cora and Sybella learn when they come across a diary detailing best-friendship problems. Told in two intertwining points of view, comes a warm, wry story of friendship, growing up, and being true to yourself. Written by Rebecca Donnelly, author of How to Stage a Catastrophe (an Indies Introduce and Indie Next List honoree), The Friendship Lie will speak to any reader who has struggled with what to hold on to and what to throw away.