Every Day by David Levithan

Every day a different body. Every day a different life. Every day in love with the same girl.
There’s never any warning about where it will be or who it will be. A has made peace with that—even established guidelines by which to live: Never get too attached. Avoid being noticed. Do not interfere. 
It’s all fine until the morning that A wakes up in the body of Justin and meets Justin’s girlfriend, Rhiannon. From that moment, the rules by which A has been living no longer apply. Because finally A has found someone they want to be with—day in, day out, day after day. But can Rhiannon love someone who is destined to change every day?