And the romance doesn't dissolve into some stupid love triangle between basically equally or more good looking guys/girls and more like the the SML or SFL is an antagonist. And the plot is more about their own decisions that might break the way to a happy ending. Kinda like how the mermaid gave away her voice to the witch, etc, like making trouble for herself, yet at the same time gave her the chance to pursue the guy she liked.

Possibly The Legend of the Blue Sea? It has a very fairytale feeling and is a lot like The Little Mermaid, and the plot is definitely about their own decisions. There's also no real love triangle just a brief crush from another girl (there is a secondary couple, though).