Praise be to Allah.
It is not permissible for a Muslim to curse his fellow Muslim, no matter what the reason, because cursing is a prayer against him, asking that he be cast out far away from the mercy of Allah. This is not something that it is permissible for a Muslim to pray for against his fellow Muslim.
In Fataawa al-Lajnah ad-Daa’imah (19/240) it says: Cursing a Muslim is a major sin, because of the proven report from Thaabit ibn ad-Dahhaak, according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: “Cursing a believer is like killing him.” End quote.
This hadith was narrated by al-Bukhaari (6105) and Muslim (110).
It is obvious that killing a believer is one of the gravest of major sins. The fact that the Prophet (blessings and peace of Allah be upon him) described cursing him as being like killing him is indicative of the gravity of the prohibition on cursing him.
See the answer to question no. 83390 .
The one who curses a Muslim has prayed against him, asking that he be cast out far away from the mercy of Allah, so if the one who is prayed against is deserving of being cursed, this prayer will be answered and the curse will befall him. If he does not deserve that, the curse will rebound on the one who uttered it.
That is because of the report from Abu’d-Darda’, who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “When a person curses something, the curse ascends to heaven and the gates of heaven are locked against it. Then it comes down to the earth and its gates are locked against it. Then it goes right and left and if it does not find anywhere to go, it goes back to the one that was cursed and if he deserved that (then it befalls him), otherwise it goes back to the one who said it.” Narrated by Abu Dawood (4905). Its isnad was classed as jayyid by al-Haafiz Ibn Hajar in al-Fath (10/467). It was classed as hasan by al-Albaani.
Based on that, what she uttered against you of curses and insults will not harm you, for two reasons:
The first reason:
The curse only befalls the one who deserves it, and the one who deserves to be cursed is the disbeliever, the innovator, and the evildoer who commits major sins.
If your mistreatment of this woman did not involve any of these things, then the curse will never befall you, and her prayer against you that you be cursed comes under the heading of transgression and wrongdoing. If the supplication of one Muslim against another includes transgression and wrongdoing, it will not be answered, as the Prophet (blessings and peace of Allah be upon him) said: “A person will still be answered so long as his du‘aa’ does not involve sin or severing ties of kinship.” Narrated by Muslim in his Saheeh (2735).
If one Muslim curses another, specific Muslim without justification, that is one of the greatest of sins, so how could one who says that receive an answer to his prayer?
The second reason:
Repentance removes sins and the impact thereof. So if someone commits a sin that is deserving of being cursed, then repents from it, the curse will no longer affect him because the reason for it has been removed.
Shaykh Ibn ‘Uthaymeen said: The sins that came before repentance are completely erased by repentance, and they will have no longer have any impact … The curse may happen at a time when one is committing sin, or it may be delayed, according to the decree and wisdom of Allah. But if Allah blesses the person by enabling him to repent, we know that this curse is ended, because what is meant by the curse is being cast out far from the mercy of Allah, but the one who repents to Allah is subject to the mercy of Allah.
End quote from Fataawa Islamiyyah (4/174).
And the Prophet (blessings and peace of Allah be upon him) said: “The one who repents from sin is like one who did not sin.” Narrated by Ibn Maajah (4250); classed as saheeh by al-Albaani.
What you must do is hasten to seek pardon and forgiveness from her for your mistreatment of her. But if going to her and talking to her will cause more problems, then you should strive to ask Allah to forgive her and offer supplication for her, and speak well of her to others.
Shaykh Ibn Baaz (may Allah have mercy on him) said: The believer should be keen to ensure that he does not wrong his brother or withhold his dues from him. So he should either give him his dues, or ask him to pardon him. If his transgression affected his honour and reputation, then he must seek his pardon if he can. If he is not able to do that or is afraid of the consequences, and what may result from telling him would be a greater evil, then he should ask Allah to forgive him and offer supplication for him, and speak well of him, mentioning his good characteristics that he is aware of, to make up for the bad things that he said about him.
End quote from Majmoo‘ Fataawa Ibn Baaz (4/375).
And Allah knows best.