She's Come Undone by Wally Lamb
OK, it has been years since I read this one, but I loved it and want to tell you about it. So I am going to rely on the blurb on the back of the book.
"Meet Dolores Price. She's thirteen, wise-mouthed but wounded, having bid her childhood goodbye. Beached like a whale in front of her bedroom TV, she spends the next few years nourishing herself with Mallomars, potato chips, and Pepsi her anxious mother supplies. When she finally rolls into young womanhood at 257 pounds, Dolores is no stronger and life is no kinder. But this time she's determined to rise to the occasion and give herself one more chance before really going belly up."
"... the most heartbreakingly comical heroine to come along in years. At once fragile girl and a hard-edged cynic, so tough to love yet so inimitably loveable, Dolores is as poignantly real as our own imperfections."