Poor unfortunate soul indeed! The original story of the Little Mermaid is a much darker (fish)tale than its Disney counterpart.
The original story is a Danish fairytale from 1837, written by H.C. Andersen. It starts out very similarly to the Disney film, you have a little mermaid princess who dreams about the world up above the surface.
She has to wait five years before she's allowed to go up to the surface, and in the meantime her sisters tell her of all the things they have seen. On her 15th birthday she finally swims up above to see the world of humans. She sees the ship, and falls in love with the prince, just like in the movie. A storm hits, and she ends up saving the prince from drowning, bringing him to a nearby beach close to a temple. She hides while the girls from the temple find the prince and take care of him.
Back down under the sea, the Little Mermaid gets completely obsessed with this guy that she's never even spoken to. She often swims up to the surface to spy on the prince which basically makes her the Belieber of the sea.
The Little Mermaid goes and speaks to her grandmother about it, who tells her she can never marry a mortal man because they think her fishtail would be ugly and without legs she could never be loved. Awesome job on giving your grand-daughter confidence issues! She also informs the teenage girl that mermaids don't have souls, so when they die they turn into sea foam. Humans have souls, and get to go to heaven. The only way a mermaid can gain a soul and go to heaven is by marrying a human in a church. Totally healthy thing to tell your grand kids really.
The Little Mermaid is so desperate to get legs that she goes to visit the Sea Witch, who lives in a house made of bones. Charming architecture! The Sea Witch is so grim she makes Ursula look like a semi-decent person.
She tells the Little Mermaid that she can totally help her get legs by making a potion. When she drinks the potion it will feel like someone cut her in half with a blade, and every step she takes with her legs will feel like she's stepping on knives. Oh, and to pay for the whole thing? She wants her voice, but not by using some magical spell where she pulls a glowing ball of light from her throat. Oh no, she wants to cut off the mermaid's tongue. And if the whole thing doesn't work out, and the prince marries someone else, she will turn into sea foam.
If the Little Mermaid was a sane person, and not an obsessed love-struck teenager, she would have said no thanks to this terrible deal. Alas, she is the Belieber of the sea and agrees to the terms. Her tongue is cut off and she gets the bottle of potion and swims up above to drink it.
She swims to the prince's palace and drinks the potion, and just like the Sea Witch foretold, it feels like someone cutting her in half. The Little Mermaid is no longer a mermaid, she has been given legs and sits naked by the palace.
The prince finds her there, takes her in and gives her clothes. He calls her his little mute "findling", which is super creepy. It is agony to walk on her new legs, but she ignored the pain even when her feet are bleeding. On her first night they are entertained by dancing slaves. She joins them in their dance, and everyone applauds her because she is super graceful. Nobody but her knows how painful it is.
The prince falls in love with her, even though they never speak a word. It's not that nice though, because the prince tells her that she reminds him of another girl he once knew, from a temple, and he would totally marry her instead, if she wasn't living in a religious temple. So you know, the Little Mermaid is his second choice and he tells her this to her face. Lovely!
No worries though, this other girl can't marry the prince due to religious obligations, so the prince agrees to marry the Little Mermaid. I don't know about you guys, but I've never heard of anything more romantic.
A new player enters the game
The king and queen of the kingdom force the prince to visit a neighboring kingdom, to meet a princess they think would be suitable for him to marry. The prince says he will go, but he will not marry this random princess. The Little Mermaid goes with him on the journey, happily believing she is soon going to marry this douche.
Well surprise, surprise! The random princess is actually the girl from the temple, the one the prince loves with all his heart. She was just being raised in a temple to learn some virtues, and now she's a princess ready for marriage. The prince marries her, and the Little Mermaid's heart is broken. She knows that this will be her last night alive, and as the sun rises in the morning she will turn into sea foam.
Her sisters are still mermaids, and they obviously don't want her to die, so they swim over to the Sea Witch to strike a deal. The Sea Witch gives them a dagger in exchange for their hair, and if the Little Mermaid stabs the prince to death with said dagger before sunrise, she will return to the sea as a mermaid.
Obviously she should go ahead and do this, the prince treated her like a piece of garbage anyways. She sneaks into his room at night, where he and his new wife are enjoying some post-coitus sleep. Unfortunately, the Little Mermaid just can't bring herself to kill the guy, so she throws the dagger into the sea and throws herself in after it. The sun rises and she feels her body dissolve into sea foam.
As she's dissolving, she begins seeing these transparent flying girls up in the sky. Turns out they are "Daughters of the Air", and she turns into one of them. They explain that they will be allowed to enter heaven if they just serve God for 300 years, and so the Little Mermaid at least gets one wish fulfilled. She flies up to the sky, and on her way she stops to give the princess, not the prince, a kiss on the head.
As you can see, it's no wonder Disney decided to change things up a bit when they adapted the story. However, the upcoming live-action movie is apparently going to be more inspired by the original tale rather than the Disney-version, so that will definitely be interesting to see!
If you need some cheering up after reading this, here's a clip from the Disney-version of the movie with the cheerful song 'Under the Sea'.