Andersen Monogatari

Anthology TV series Andersen Monogatari (Andersen Stories) is a 1971 anime that adapts a number of Hans Christian Andersen's stories, some of them within a single episode, and some over multi-episode arcs.

The Little Mermaid got a three-episode adaptation, which is practically movie-length, and although it's a serious take that sticks to the melancholy, it features quite a rebellious little mermaid who initially needs to be locked up after she secretly swims up to the surface without being allowed to. The mermaid knows her own mind and goes for it! Even though in this case she ends up heartbroken as per the original story.

This adaptation also features a prince who is particularly obsessed with finding the beautiful girl who saved him. When he sees the other princess the first time, he is enchanted by her beauty but admits that he still wants to find the mysterious girl who saved him, and it's the little mermaid's mistaken attempt to tell the prince the truth that has the prince misunderstanding and thinking that his princess was the one who saved him. Tragic!

Watch the episodes on youtube (no subtitles though):  episode 1 ,  episode 2 , and  episode 3