Praise be to Allah.
You have to repent to Allaah from your friendship with this woman, for this sin that you have fallen into came about as a result of your taking lightly the matter of this relationship and of being alone with women. This is an act of disobedience towards Allaah which is deserving of His punishment and torment. Concerning the seriousness of this matter, please see Questions # 1114 and 9465.
Secondly: make a final end to your relationship with this woman and with any other women with whom you have a relationship, because most of these relationships end up in committing zinaa which is haraam, or some other kind of physical relationship which is haraam – we seek refuge with Allaah – even if at the beginning it was, as you say, a chaste relationship. For the Shaytaan flows through the son of Adam like his blood. Know that being alone with a non-mahram woman can never be described as a chaste relationship.
Now you must hasten to repent to Allaah sincerely, by regretting what has happened in the past, giving up this relationship and sincerely resolving never to have any haraam relationship again. This evil woman is trying to make you imagine and to convince you that you committed an immoral action with her so that she may use that as a means to make you commit immoral actions with her again. Even if what she claims were true, that you committed a haraam action with her, do not let the Shaytaan make the most of this opportunity or make you despair of the mercy of Allaah, so that he can mislead you further by making you feel that doing this again and persisting in it is not a serious matter, or make you think that repentance is too difficult. The Shaytaan is eager to make you feel this way, but the mercy of Allaah is vast, so hasten to repent. Allaah says (interpretation of the meaning):
“Say: ‘O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful”[al-Zumar 39:53]
Allaah forgives the sins of those who sincerely repent. Allaah says (interpretation of the meaning):
“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;
Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft Forgiving, Most Merciful[al-Furqaan 25:68-70]
It was narrated from ‘Abd-Allaah ibn Mas’ood that a man kissed a woman. He came to the Prophet (peace and blessings of Allaah be upon him) and told him about that. Then the aayah was revealed (interpretation of the meaning):
“And perform As Salaah (Iqaamat as Salaah), at the two ends of the day and in some hours of the night [i.e. the five compulsory Salaah (prayers)]. Verily, the good deeds remove the evil deeds (i.e. small sins). That is a reminder (an advice) for the mindful (those who accept advice)”[Hood 11:114]
The man said, “is this for me, O Messenger of Allaah?” He said, “It is for all those of my followers who encounter a similar situation.”
According to another report: “a man did something with a woman that was less than intercourse.” (narrated by Muslim, al-Tawbah, 4964)
Do a lot of righteous deeds, prayer and seeking forgiveness. Look for good, religiously-committed friends who will offer you an alternative to these haraam relationships. Know that the gate of repentance is open until the sun rises from the west, and that Allaah accepts the repentance of His slave until the death rattle sounds in the throat.
Finally, you must hasten to take the means prescribed in sharee’ah to protect yourself – by Allaah’s leave – i.e., marriage, which will keep you from falling into haraam,.
May Allaah help you and us to do that which He loves and which pleases Him. May Allaah bless our Prophet Muhammad.