Open Waves by Viola Grace

A rejected mermaid heads to the islands to heal her soul, finding that the support of family and friends is almost as good as finding her sea monster. Almost.

After seeing how happy some of the other mages can be with their elves, Meeda decides that returning to her home is probably the best course of action. Seeing her destined mate swim away from her broke something inside her. The waters of the islands soothe that bit of her shattered soul.
Having the resources to get away from it all is nice, and having an understanding older sister is better. With dark elves moving to the islands, it makes sense that the sea monster who got away is there, and to Meeda’s surprise, a cultural difference caused him to leave her behind. She had no idea she was supposed to chase him.