One possible reason could be their fundamental differences in personality. Harry is brave and impulsive at times, while Snape is often brooding and secretive. Their past interactions, which were mostly filled with animosity and mistrust, could also be a factor. Harry might not be able to see Snape in a romantic light due to all the times Snape was cruel to him in school.
The original Harry Potter story focuses on Harry's battles against Voldemort and his growth as a wizard. The fanfiction about Harry refusing to marry Snape takes a different turn into the realm of romantic relationships which were not central in the original. Also, in the original, the relationships are more along the lines of friendship, mentorship, and enmity, not romantic love between these two characters.
She might be heartbroken. Ginny could initially be in shock and disbelief. She may retreat into herself, spending a lot of time alone in her room, crying and trying to understand why Harry would refuse.
One possible reason could be Snape's lingering resentment towards James Potter, Harry's father. Snape might see Harry as a reminder of the bullying he endured from James during their school days. So, he takes out his anger on Harry.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.
Ginny could be heartbroken. She might have expected to marry Harry after all they've been through together. She could initially go into a state of shock and disbelief, not understanding why Harry has refused.
One possible reason could be that Harry has seen so much death and destruction during the war that he is afraid of forming new, deep emotional bonds. He might worry that he could lose his loved ones again, so he refuses to marry as a way to protect himself from potential future heartbreak.
In this fanfiction, Harry might stand up to Snape's abuse by using his wits. He could expose Snape's unfair treatment in front of the other professors. For example, he might gather evidence of Snape's partiality in grading or his excessive use of punishments towards Gryffindors. By presenting this evidence to Dumbledore or other authority figures, Harry would be taking a stand against Snape's abuse.
One possible reason could be that Harry is still haunted by his past traumas and feels he's not in the right mental state to marry anyone. His experiences with Voldemort and the losses he's endured might make him reluctant to bring Ginny into a potentially unstable future.
Maybe Harry has some unresolved issues from his past, like the trauma of losing his parents and many of his friends during the fight against Voldemort. These could make him hesitant to enter into a marriage with Ginny. He might also be worried about how the wizarding world would view their marriage, especially if there are still some who oppose him or his family.