Is the questioner considered sinful in her stance toward her father after learning of his illness, and in all the difficult circumstances she went through, especially after her perception of him changed as a result of her mother's talk about her, and the hardships the family endured?

It is not permissible to abuse or disown one's father, no matter how much he may err. Rather, it is obligatory to honor him and accompany him with kindness. The expiation for any previous neglect or mistreatment is repentance, which involves abandoning the sin, regretting it, resolving not to return to it, and seeking forgiveness from the father. Allah accepts repentance and pardons evil deeds.