There are a few things Grace knows for sure. One is that nothing will come between her and her best friend, Kya. They have a pact. Buds before studs. Sisters before Misters. But in the summer before senior year, life throws out challenges they never expected. And suddenly the person who's always been there starts to need the favour returned. Grace and Kya are forced to question how much a best friend can forgive, and the answer is not what they expected.How I Lost You is a complex look at friendships and secrets, at dealing with pain and moving on, at knowing when to help when it's difficult to stay away. Friendship ties teens together, but any kind of strain on that connection is hard to deal with, especially when the source of that strain is an extremely painful secret.Grace has goals for after high school, for college. Paintball is her life, she'd give anything to make a college team, but Kya is also her life. They share everything. Buds before studs, as Kya puts it. But Kya has her demons, her secrets, her way of coping that threatens their future plans, and it's getting harder and harder for Grace to stick by her. Because of Kya, Grace is something of a mother figure, a caregiver, but sometimes Kya doesn't want her help and goes off on her own, leaving Grace to deal with her aftermath. Some parts of Grace's life are also changing, separating the two of them. It's hard for Grace, but also for Kya. Both aren't quite sure how to move on from this secret.This book highlights how complicated friendships can become, especially those between teenage girls. Once teen girls become friends, they will stay friends for the longest time, or they'll try to unless something big happens. Grace and Kya are always together, they're been friends for years, they've made plans to be together after high school and to be friends forever. But something big and secret hangs over them, weighing one of them down, and the other could be forced to make the hardest decision: to stick with her, even though it could mean being pulled down into her dark world, or to let go and live her life, continuing with her dreams.As much as this is about friendship, it's also about moving on. About coping, about accepting. Kya has the darkest secret, suffers from the darkest pain, and struggles to find a way of moving on. Grace finds her method to be unconventional, to be dangerous, and wishes Kya would stop changing.Heartbreaking and emotional, both girls also have their exciting moments. Both come alive when they compete together, when they are together, but something is slowly pushing them apart, slowly pushing them to accept that both of them are changing. Great insight into teenage girls and how valuable that best friend really is.