Her Steadfast Cowboy by Danae Little

A lonesome cowboy, his best friend with no memory, and danger chasing her back into his arms.
With city-girl clothes and a cowgirl attitude, Kacey Daring leaves the danger following her far behind as she returns to her small Wyoming hometown and the man she had tried to forget.
Sawyer West had been Kacey’s best friend for as long as he remembered. Yet, she rode away from him without looking back, hardening his heart and leaving trust in the dust.
After a mysterious attack leaves Kacey with amnesia, Sawyer has no choice but to sacrifice his heart to protect her from the trouble that follows.