Artichoke's Heart by Suzanne Supplee.
"It's not so easy being Rosemary Goode and tipping the scales at almost two hundred pounds - especially when your mother runs the most successful (and gossipiest!) beauty shop in town. After a spectacularly disastrous Christmas break when the scale reaches an all-time high - Rosemary realizes that things need to change. (A certain basketball player, Kyle Cox, might have something to do with it.) So begins a powerful year of transformation and a journey toward self-discovery that surprisingly has little to do with the physical, and more to do with an honest look at how Rosemary feels about herself."