Maybe in Another Life by Taylor Jenkins Reid, 2015

A what-if/choose your own adventure story based on whether protagonist Hannah Martin stays out late with her high school sweetheart Ethan or return home, Maybe in Another Life kicks off with Hannah returning home to Los Angeles at the age of 29 after a few years in New York.

We watch how Hannah's life unfolds based on a seemingly trivial decision.

By staying out with Ethan, Hannah & Ethan rekindle their romance and she soon discovers that she is pregnant, a result of her affair with a married man, one of the reasons she fled New York.

Alternatively, by returning home early with her best friend Gabby, Hannah ends up in a car accident that causes her to lose her baby and realize that she needs to focus on making mature, adult decisions as she gets better.

In both versions of Hannah's life, she ends up doing what she is meant to do, becoming a nurse; while whom she ends up "happily ever with" differs. 

It's an interesting statement that there is not necessarily one person whom one is meant to be.  While for Gabby, there is one person with whom she is meant to be. 

This idea of a trivial decision in your 20s shaping the rest of your life is further explored in the next book I read, Amor Towles' Rules of Civility.