Once and Future Wife by Clarissa Kae

Immortal men like Rhys Glyndwr belong to forgotten fables—along with his reincarnating wife—but instead, they haunt the mind of a modern scientist, Isla Belle Thorne. Like an old blanket, visions of the healer servant and his wife, the daughter of a duke, have comforted Isla Belle, the only constant in her unstable childhood.
When her mother is hospitalized for a mental breakdown, Isla Belle fears for her own health and keeps the visions a secret. As Isla Belle starts her new job at a renowned medical organization, she comes face to face with Rhys—the same man from her visions.
She’s told an impossible tale of a love that death cannot kill. Surrounded by science but faced with an implausible legend, Isla Belle must decide what to believe and what to leave behind.