Maybe in Another Life by Taylor Jenkins Reid
"At the age of twenty-nine, Hannah Martin still has no idea what she wants to do with her life. She has lived in six different cities and held countless meaningless jobs since graduating college. On the heels of leaving yet another city, Hannah moves back to her hometown of Los Angeles and takes up residence in her best friend Gabby’s guestroom. Shortly after getting back to town, Hannah goes out to a bar one night with Gabby and meets up with her high school boyfriend, Ethan. Just after midnight, Gabby asks Hannah if she’s ready to go. A moment later, Ethan offers to give her a ride later if she wants to stay. Hannah hesitates. What happens if she leaves with Gabby? What happens if she leaves with Ethan? In concurrent storylines, Hannah lives out the effects of each decision. Quickly, these parallel universes develop into radically different stories with large-scale consequences for Hannah, as well as the people around her. As the two alternate realities run their course, Maybe ...