One possible fault could be that the character development wasn't as deep as it could have been. It felt a bit rushed at times, leaving you wanting more depth.
I'm not sure specifically what your 'my fault full story' is about. Maybe it's about a mistake you made? If you could give me some more details, it would be easier to answer.
One popular 'Your Fault' Harry Potter fanfic could be about Harry being blamed for a magical mishap that wasn't really his doing. For example, if there was a spell gone wrong in Hogwarts and everyone wrongly accused Harry because of his reputation. Another might focus on a situation where Harry is made to feel guilty for a decision Dumbledore made, like in a situation where a plan backfires and Harry is seen as the one at fault. There could also be fanfics where Harry is wrongly accused of stealing a powerful magical artifact, when in fact he was trying to protect it.
First, think of a situation where Harry is wrongly accused. It could be something as simple as being blamed for a broken school rule he didn't break. Then, develop the characters' reactions. For example, how Ron and Hermione would defend him. Next, create a plot around this false accusation. Maybe Harry has to prove his innocence while also dealing with the school's punishment. Also, add in some magical elements like spells or magical creatures to make it more interesting.