Return to the Deep by P. Jameson

Huran found his mate, his cursebreaker, his beloved, long ago. His happily-ever-after should be in the bag. But he’s been keeping a secret from her: the curse that turned him into a finned wonder was never completely broken. Magical kisses be damned, something holds him to the water. And he’s not sure how long he can keep his lingering Mer-tail from the people he loves.
Christienne knows Mer lore like she knows the shape of her mate’s abs. But something is very wrong with Huran. When she catches him sneaking out to the lake at night, she suspects his secret is bigger than she can handle. Somehow, she must find a way to meet her merman in the depths of his despair if there is any hope of them making it to their next mate-aversary.
But she has a secret of her own, and no guarantee that Huran will be by her side when the truth comes out. There’s only one thing to do now…
Fight for love and hope it’s enough.