He might be initially heartbroken. Harry has always been a bit of an emotional person, so he could go into a state of depression for a while. He might isolate himself from his friends and focus only on his studies or fighting against the dark forces, using it as a way to distract himself from the pain of the rejection.
One reason could be that his mate has different life goals. Maybe she is more focused on her own magical career and doesn't see a relationship with Harry fitting into that at the moment. Another possibility is that there are misunderstandings between them. For example, Harry might have been too involved in some dangerous adventures and his mate thought he didn't value their relationship enough. Also, it could be due to external pressures, like her family or friends not approving of Harry for some reason.
He could also give his mate some space for a while. Sometimes, distance can make the heart grow fonder. By giving space, his mate might have time to think about their relationship and realize what they are missing.
It could also be due to differences in their personalities or goals. If Harry is very adventurous and his mate is more cautious, they might not see eye - to - eye on certain important matters. And this fundamental difference could lead to the mate rejecting Harry.
One possible reason could be a misunderstanding. Maybe his mate misinterprets something Harry does or says. For example, if Harry is often away on dangerous adventures, his mate might think he doesn't care about their relationship and reject him.
Maybe it's Ron. Suppose Ron gets jealous of Harry's fame and power and, in a moment of weakness and envy, decides to abandon Harry to die. This would create a lot of drama as Ron and Harry have been friends for a long time, and such a betrayal would be a huge shock to the story's readers.
It could be the Death Eaters. Even if they are on the 'bad side', they might see an opportunity to disrupt Harry's life and weaken him. By forcing him to reject his mate, they can create emotional turmoil for Harry, which would make him less focused on fighting against them.
Well, without actually reading the specific fan fiction, it's hard to say precisely. But generally, it might involve Harry having feelings for his mate and then misinterpreting some actions or words as rejection. Maybe his mate is just shy or dealing with their own issues.
One possible reason could be some sort of external threat. Maybe there's a dark force that wants to use his mate against him, so to protect her, he's forced to reject her. For example, if there are dark wizards who plan to capture his mate and use her as a bait to get to Harry, he might think rejecting her would keep her out of danger.