The Healer by Frost Kay

Nyx has been in love with the same man for as long as she could remember. She’s tried to tame his demons, heal the hurts of the past, but he keeps her at arm’s length no matter what. When her last attempt to sway him to her side fails epically, Nyx decides to walk away from her mate and move on.
Dark memories from the past have always haunted Briggs. He’s never been able to trust himself with anything precious—especially his mate. Too much blood stained his hands to take Nyx as his wife, so he must settle as her friend and protector. But it’s not enough for either of them. When she leaves him behind for good, he knows that he’s made a mistake and can’t let her slip through his fingers. It’s time to put the past when it belongs and fight for the only woman he’s ever loved.