The Two Lives of Lydia Bird by Josie Silver

 Lydia has been in love with Freddie since they were teens. Now they're engaged. She's waiting for him to pick her up for her birthday dinner when he calls and says he has to swing by his friend Jonah's and pick him up, which will make them late for dinner. Then the police show up and say there's been an accident and Freddie has died. Lydia is devastated. Her doctor gives her some sleeping pills to get her through the first traumatic weeks and she discovers that the pills give her access to an alternate world, where Freddie is still alive.  After  a while she realizes that in the alternate world, life has gone on. Alternate Lydia has picked out a wedding dress and venue that Awake Lydia has never seen. And Freddie keeps commenting that she has changed. It's a compelling story of how Lydia learns to deal with grief and eventually get the strength to move on with her life without Freddie.