Likely answer

Novel About a Girl Living Alone in the Marsh and a Broken Promise

Delia Owens's bestselling novel Where the Crawdads Sing follows Kya Clark, who raises herself alone in the North Carolina marshes. Her life is shaped by her romance with Tate Walker, who promises to return to her, and the later mysterious death of Chase Andrews that entangles her in a murder investigation.

Confirmed match: Where the Crawdads Sing by Delia Owens