Perhaps Snape uses his position of power over Harry at Hogwarts. He could give Harry an unfairly low grade on an important assignment that Harry worked really hard on, and then mock him for his failure. This public humiliation and disappointment could break Harry's heart, especially if Harry had been looking up to Snape in some way.
One possible plot could be that Snape reveals some long - held secret about Harry's parents in a very cruel way. For example, he might tell Harry that his father was not as heroic as he thought, and this shatters Harry's idealized view of his family, thus breaking his heart.
In the fanfiction, Snape might break Harry's heart by choosing the side of the Death Eaters over Harry in a crucial moment. Harry might have thought that Snape had changed or was on their side, but then Snape's betrayal would be a huge blow to Harry. This would not only break his heart but also make him question his own judgment about people.
Perhaps Tonks starts dating someone else, like Remus Lupin. Harry has some unspoken feelings for Tonks, and seeing her with Lupin makes him feel heartbroken. In the fanfiction, it could show Harry's internal struggle to accept the situation and his pain at seeing the woman he cares about with another man.
Well, it could be that Edward, perhaps jealous of Harry's popularity or a girl he likes, spreads false rumors about Harry. This causes Harry to be ostracized and his heart to break as he loses friends and respect.
The relationship would become extremely strained. Harry would likely go from having some level of respect or at least tolerance for Snape to feeling intense hatred. He might avoid Snape at all costs and even start to plot revenge in his head.
In this fanfiction, it could be that Akeno chooses someone else over Naruto. She might publicly reject Naruto in a really humiliating way, like in front of all their friends and allies. This would definitely break Naruto's heart as he is a character who values relationships deeply. He would be heartbroken not just because of the loss of a romantic interest but also because of the feeling of being publicly shamed and rejected.
Well, it could be that Anko does something really cruel like betraying Naruto's trust. Maybe she lies to him about something he deeply cares about, like his dreams of becoming Hokage. And this lie shatters his hopes and thus breaks his heart.
Well, it could be that Annabeth betrays Percy's trust. Maybe she lies to him about something really important, like her true feelings or her actions. This lie could be the start of breaking his heart.
Well, it could be that Artemis, in the fanfiction, has to make a choice that goes against Percy's feelings. Maybe she has some divine duty that forces her to break his heart. For example, she might have to sacrifice their relationship to protect her hunters or to maintain some sort of balance in the mythological world.
It could be that Sonic makes a comment about not believing in true love or commitment. Since Amy is so deeply in love with Sonic and dreams of a future with him, this comment would be like a dagger to her heart. She might then question her own feelings and the relationship she thought she had with Sonic, leading to a heart - breaking situation for her.
Well, it could be that Hinata starts to ignore Naruto for some reason, like she falls in love with someone else. Maybe she gets influenced by some external force or false information that makes her believe Naruto isn't good for her. This sudden change in her behavior towards Naruto would break his heart as he might have deep feelings for her.
Well, it could be that Hinata might break Naruto's heart by choosing someone else over him. Maybe she realizes she has feelings for another character, like Sasuke perhaps. This would be a huge shock to Naruto as he has always seen Hinata as a special person in his life.