One possible reason could be jealousy. Maybe Harry has achieved something great and his friends can't handle his success. For example, if he gets a very important role in a magical competition, his friends might feel overshadowed. Another reason could be a misunderstanding. Harry might be wrongly accused of something, like being in league with the dark side, and his friends, in their fear or haste, abandon him without giving him a chance to explain.
Perhaps it's due to external influence. A powerful and malevolent force could be manipulating his friends' minds. This force might be spreading false rumors about Harry to drive a wedge between them. It could also be that his friends have their own personal issues to deal with. For instance, one of them might have family problems that require their full attention, and in the process, they unknowingly neglect and eventually abandon Harry.
In some cases, it could be a test of loyalty. The wizarding world might be facing a hidden threat, and Harry's friends are made to believe that abandoning him is for the greater good. However, this is a cruel test that leaves Harry feeling betrayed. It's also possible that over time, his friends' values have changed. They might have started to pursue different goals in life that don't align with Harry's, leading to their separation.
Harry can focus on self - improvement. He can pour his energy into his studies, become an expert in different branches of magic. This would not only boost his confidence but also make him more independent. For instance, if he masters advanced potions making, he'll feel a sense of accomplishment on his own.
He would probably try to figure out what went wrong. Harry is a curious and determined character. He might start to recall recent events to see if there were any signs of his friends' discontent. He could also try to reach out to them, either directly or through mutual acquaintances, to ask for an explanation.
One possible plotline could be Harry feeling extremely depressed at first. He might isolate himself even more, spending long hours in the library or wandering the Forbidden Forest alone. Another plotline could involve him finding new, unexpected allies who help him grow stronger and more independent. For example, he could befriend some of the house - elves who have their own unique magic and knowledge.
He might turn to his studies more intensively. Since he has no friends to distract him, he could pour all his energy into learning new spells and magic, becoming more powerful in the process.
One reason could be a misunderstanding. Maybe there was a miscommunication between Harry and his friends, and he felt so hurt or angry that he decided to leave. For example, if his friends wrongly accused him of something he didn't do.
Maybe he is new in the place and has not had the chance to make friends yet.
One reason could be that the author lost interest. Maybe they started with a great idea of a female Harry Potter but then got distracted by other things in life or new fanfiction ideas. Another reason might be that they received negative feedback which demotivated them from continuing.
One possible storyline could be that Harry discovers his friends have been influenced by dark magic or a powerful spell that makes them shun him. He then has to find a way to break the spell while also dealing with his own feelings of betrayal and isolation.
It could also be that Harry himself is so traumatized by the war that he isolates himself. He might push people away, thinking that he's a danger to them. As he withdraws, others might misinterpret it as him not wanting their company and gradually abandon him.
Another reason might be jealousy. If Harry suddenly gains more power or attention in the fanfiction world, his friends could become jealous. For example, if he discovers a new and powerful magic spell, and all the focus in the magical community is on him, his friends might start to feel overshadowed and distance themselves from him.