L is for Life on Hold by Beverly Stowe McClure

Blurb (from amazon.com): A paper found. A secret revealed. A girl’s life changed forever. Myra Gibson’s life is a lie. For sixteen years her parents have kept their secret, but the adoption paper she discovers while cleaning the summerhouse tells the truth. As the past and present collide, Myra finally stands up for herself and begins a journey she may regret.