Still in Love is a heartbreaking character who doesn't get to be happy even in fanwork of her.
In both canon and fanon, her "other side" is more loved and focused on while the true Still never gets to exist.
A story of a teenage girls struggle with mental health from a young age gets sidelined for a made-up plot of a "yandere who is madly in love with her trainer."
The truth is Still and her "other side" do not fit the yandere trope aside from being mentally ill and having an obsession. They do not act lovey-dovey/innocent with the trainer nor do they get lost in obsessive love to the point of mental illness and/or extreme violence. Still's mental illness has existed since childhood, due to her being buliied by children and adults alike for her "other side," and the only obsession they have is her "other side" wanting to win.
Still can't leave her trainer because they're the only person to accept her "other side" and not treat her horribly for it, but her trainer isn't innocent. And that leads to Still either being consumed by her untreated mental health issues to the point of ending her own life or her "other side" takes over and the real Still in Love is forgotten.
And the game dares call the second ending her "good" ending. Still in Love does not get a good ending. Not even in fanon. Most fanart of her is her "other side," most animations of her are her "other side," everything is about her "other side." This "other side" that Still in Love despises and does not want.
Still in Love is a character that, in both the game and in real life, was never allowed to simply exist and have a good ending. So much so that people even spread misinformation about and sexualise the real life horse being "in love" with her jockey. She was inbred and had behavioural issues due to it. She had one child before suffering from complications and, instead of people trying to spread awareness on the horrors she went through from even before her birth, people have the balls to say she "died because she couldn't handle having anyone's child but her jockeys." it's disgusting how horribly her in-game and real-life self is treated by a fandom who claims to care about these horses and characters.
Still in Love is not a "yandere horse girl." And I will die on this hill.