As She Fades by Abbi Glines

On the night of her high school graduation, Vale McKinley and her boyfriend Crawford are in a terrible car accident that leaves Crawford in a coma. They were supposed to spend the summer planning for college, for a bright future full of possibility. Together. Instead, Vale spends long days in the hospital, hoping Crawford will awaken.
Slate Allen, a college friend of Vale’s brother, has been visiting his dying uncle at the same hospital. When he and Vale meet, she can’t deny the flutter of an illicit attraction. She tries to ignore her feelings, but she’s not immune to Slate’s charm. Slowly, they form a cautious friendship.
Then, Crawford wakes up . . . with no memory of Vale or their relationship. Heartbroken, Vale opts to leave for college and move on with her life. Except now, she’s in Slate’s territory, and their story is about to take a very strange turn.