Where We Belong by Emily Giffin

Marian is a successful producer who has all but forgotten her years in high school until a young woman named Kirby knocks on her door in the middle of the night. At first Marian is confused, but as she takes in Kirby's appearance she realizes exactly who she is. Where We Belong is not only a story of finding out who you are, it's a story of redemption, honesty, and (above all else), love.