Right Here, Right Now by Shannon Dunlap

Worlds turn. Particles spin. Love endures.
 
There are infinite universes in which Elise never dies. Her best friend, Anna, never has to mourn her or choose between the weight of her grief and the weight of her ambition. Her cousin, Liam, never has to lose another loved one or fight to find purpose in a life that already doesn’t feel like his own. 
 
But Liam and Anna do not get to choose the universe in which they live. Across multiple worlds, their paths collide as they wrestle with what it takes to save someone else and how to face love and loss on a quantum scale.
 
This moving, lyrical novel introduces two teens on the cusp of finding out who they are while finding each other again and again.