How should a girl act when she has become attached to a person and promised him future marriage, while her father has sworn to divorce her mother if she marries this person?

You have done well by repenting, and Allah accepts it. Allah's forgiveness is not overwhelmed by any sin: "Indeed, Allah forgives all sins." One must be cautious about being lenient in similar relationships, and keen on acquiring knowledge, performing righteous deeds, and associating with righteous women. A father does not reject a competent suitor, but he might have seen something in the young man that prevents him from marrying off his daughter to him, especially if he was the cause of his daughter's corruption. If the father insists on refusing, then his obedience is obligatory, and obedience to the father takes precedence over marrying this young man. The father's oath by divorce confirms the necessity of avoiding marriage to him. Forgetting the young man is possible by seeking Allah's help, supplicating, fearing Him, and having sincere resolve, knowing that love marriages can fail, and perhaps there was good in the prohibition.