Praise be to Allah.
Feelings of sin and regret for what one has done are signs of divine care for the one who committed the sin and serve to motivate him to follow the right path after having gone astray. We do not want to give the impression that what you have done is something insignificant, rather you should realize that what you have done angers your Lord and that it is a major sin. But we want to show you the path of repentance and let you know that even though Allaah is not harmed by your sin and has no need of your repentance, still He has helped you to repent and He rejoices at your repentance.
You have to give up your previous sin, and sincerely regret what you have done, and resolve firmly never to go back to this sin.
You should never despair of the mercy of Allaah, for Allaah forgives all sins. All you have to do is to repent sincerely and after that you will see that which will bring you joy; and seek the pleasure of Allaah.
You also have to end your relationship with this woman lest you go back to sinning. The basic principle for the repentant Muslim is that he should keep away from all environments in which he used to disobey Allaah and he must avoid and shun all the means that could lead him to fall into sin.
So we do not think that you should get back in touch with this woman, even with the intention of calling her to Islam. You have to save yourself first. Who can guarantee that if you go back to sin – Allaah forbid – you will be enabled to repent again?
You could ask some trustworthy Muslim women to contact her and call her to Islam. We do not think that you should be the one to do that.
We do not think that it is permissible for you to marry her as she is at present, not because she is a Christian, but because she is not chaste – according to what you say. Allaah has permitted us to marry women from the people of the Book, but that is subject to the condition that they be chaste, i.e., they do not commit zina or have boyfriends.
What we advise you to do is to marry a religiously-committed Muslim woman who will help to protect your religious commitment and help you to do good and to obey Allaah. This is the way to success.
And Allaah knows best.