Is it permissible to supplicate against the oppressor, and does that entail sin, and what should a woman do who supplicated against her fiancé for his injustice then regretted it, and would forgiveness and pardon have been better?

It is permissible for the wronged to supplicate against the oppressor to the extent of the injustice, without transgression. If one exceeds this limit, they become an oppressor themselves, and their supplication will not be answered. The annulment of an engagement is not necessarily an injustice; it could be due to a legitimate reason. Forgiveness is better than seeking revenge for oneself, as Allah Almighty says: "And if you are patient, it is better for those who are patient," and "Let them pardon and overlook." The Prophet (peace be upon him) also said: "Allah does not increase a servant by pardoning except in honor."