Percy might be devastated. He could become withdrawn and moody, shutting out his friends and allies. He might also be filled with anger and a sense of betrayal, which could lead him to make rash decisions.
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.
In a 'artemis breaks percy's heart fanfic', it could be that Artemis is deceived by someone into believing Percy has done something wrong. As a result, she distances herself from him, breaking his heart. Percy then has to prove his innocence while dealing with the pain of her rejection. This could involve a lot of emotional turmoil for both characters, with Percy trying to win her back and Artemis struggling with her feelings and the false information she was given.
Sorry, I'm not sure of a specific 'artemis breaks percy's heart fanfic' to recommend right off the bat. You could try searching on popular fanfic platforms like Archive of Our Own. There are usually a lot of fanfics there and you might be able to find one with that theme.
She could also be angry. Given her power and status, she might feel slighted by Percy. She may take some form of revenge, not in a malicious way but perhaps by making Percy's life a bit more difficult during his quests. For example, she could send her hunters to interfere or withhold some information that could be useful to him.
One possible reason could be differences in their duties. Artemis is a goddess dedicated to chastity and the hunt, while Percy has his own quests and loyalties in the demigod world. Their paths might have diverged so much that it led to the break - up. Maybe Artemis felt that Percy's presence was starting to interfere with her commitment to her hunters or her own divine responsibilities.
At first, Percy could be in denial. He might think that there's some misunderstanding, that Annabeth doesn't really mean to break his heart. So he tries to talk to her, to make things right. But as the truth sinks in, he could be really depressed. He might lose his appetite, not be able to sleep well, and just generally seem like a shell of his former self. His whole world revolved around Annabeth, and now that it's falling apart, he doesn't know how to cope.
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.
Perhaps it's due to the interference of the gods. The other deities might not approve of their relationship for various reasons. Maybe Zeus, fearing the power that could come from a union between Percy and Artemis, decides to put an end to it. In the world of Percy Jackson, the gods often meddle in the affairs of mortals and demigods, so this could be a factor.
One possible plot could be that Percy makes a big mistake like getting involved with another girl out of misunderstanding or being forced into a situation that makes it seem like he's cheating on Annabeth. This would shatter Annabeth's trust and break her heart.