It could be Hermione. In some alternate interpretations, there could be a situation where Hermione, while trying out a new and powerful magic, accidentally hurts Harry. Their friendship is put to the test as Harry has to deal with the pain and Hermione has to come to terms with what she's done.
In this fanfiction, it might be that his mate has some hidden secrets. When these secrets are revealed, it causes a situation where Harry gets hurt. For example, his mate might be related to an enemy in a way Harry didn't know. This revelation leads to a conflict and Harry ends up getting injured in the process. Harry then has to decide whether to forgive his mate or not based on the circumstances.
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.
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.
Yes, there are quite a few. You can start by looking on fanfiction platforms like Archive of Our Own. Some stories might explore an alternate universe where Harry and Lucius have an unexpected bond, perhaps in a world where the lines between good and evil are more blurred. It often involves complex character development and unique plot twists.
They could have an adventure exploring the various regions of Middle - Earth. Maybe they go on a quest to find a lost magical artifact that has the power to bridge the gap between the Harry Potter world and Middle - Earth.
One common theme is the exploration of forbidden love. Since Harry and Lucius are on opposite sides in the original story, their relationship in fanfiction often challenges the norms of the wizarding world. Another theme could be redemption. Lucius might be seen as redeeming himself through his relationship with Harry.
Harry might be deeply hurt. He may initially be in denial, not believing that Severus is rejecting him. After all, he may have felt a strong connection. So, he could try to reach out to Severus multiple times, hoping to change his mind.
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.