I read this fairly recently, in the last 5 years. A means of sending messages into the past was discovered, and the government decided it should be available to everyone in limited form. The societal effect was that everyone got one chance to send a message into the past, to themselves; by convention, you sent it when you were 40 (30? 50?) to when you were graduating high school. The viewpoint character was a counselor, who had never received a message from herself, and who counselled those few kids who did not receive messages.
Some people don't get messages because they choose not to send them. Others don't get messages because they don't survive to age 40. There was also discussion of whether your message could change your past and thus launch a new timeline where you didn't send that message (or any message at all).