Yesterworld by Rebecca Phelps

In the sequel to Down World, Marina is lured back through the doors by a mysterious man. ""Time after time I come back to this cold brick door. Sealed, as it has been for over a year now, as I know it must remain forever. I don't know why I keep doing this to myself. I know I can never open the door to Yesterday again. I can never have Kieren back. Never talk to Brady about the things we saw under the lake all those months ago. And never tell my brother Robbie how my heart broke every day that he was trapped in the abyss lurking behind those immovable bricks. Maybe I come to this room the way some people visit gravestones. The three doors are all I have left of my loved ones now. The rough, hard rectangles and their merciless mortar, staring back at me. Yesterday. Today. Tomorrow. And me in the waiting room. To open any of these doors is to open a pathway to death. I know this. And yet, they call to me.""Eighteen months have passed since Marina O'Connell first discovered the doors, and her life changed forever. She's a senior now. And she's living in a plane of reality with her stepmom and her dad, while Robbie and her mom are in Oregon. She and her best friend Christy are making plans for college. They're crushing on Mr. Martel, the new history teacher.