One possible reason could be miscommunication. Maybe they didn't understand each other's feelings and needs clearly. For example, Harry's preoccupation with his fight against Voldemort might have made Cho feel neglected.
The story could see Harry focusing more on his friendships. He might spend more time with Ron and Hermione, strengthening those bonds. For example, they could go on more adventures together in search of Horcruxes.
One possible reason could be their different career paths. Harry might be more focused on Auror work which is dangerous, and Hermione, being the practical person she is, may not want to live in constant fear for his safety. Another reason could be a growing apart in their values. Maybe as they get older, Hermione realizes that Harry has a more impulsive nature when it comes to making decisions regarding the wizarding world's politics, while she prefers a more calculated approach.
There's also 'Fractured Bonds' which is quite interesting. In this fanfiction, the break - up is caused by a misunderstanding that gets blown out of proportion. It shows how pride and miscommunication can lead to the end of a relationship. Hermione is heartbroken and Harry realizes his mistake too late. It's a really engaging read if you like exploring complex relationship dynamics.
I'm not sure specifically if there is a fanfiction titled exactly like that, but in the Harry Potter fanfiction community, all kinds of relationship - based plot twists are explored. It could be that someone has written a story where Cho cheats on Harry. You can try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net to find out.
There could also be a chance for reconciliation in the future. Maybe they both realize that they made mistakes during the break - up. Draco could change his ways a bit more and Harry could be more understanding. Fanfiction often has these kinds of arcs where characters get a second chance at their relationship after some growth and learning.
Ginny might be initially heartbroken. She could lock herself in her room and cry for days. After all, she loves Harry deeply. But then, she might become angry and start to focus on her own life more. For example, she could throw herself into her Quidditch career or her studies at Hogwarts.
Some in the wizarding community might be shocked. Harry is a hero, and they expected him and Hermione to be together forever. There could be a lot of gossip and speculation. People might take sides, with some blaming Hermione for breaking Harry's heart.
Harry could be heartbroken. He might isolate himself for a while, not wanting to see anyone or do anything. Just staying in his room, crying and reminiscing about the good times with Edward.
In some fanfictions, Harry might become very angry. He might feel that Ginny is being unfair or not understanding his situation. This anger could lead him to say things he doesn't really mean in the heat of the moment. However, deep down, he's really hurt and confused.