He has repented from a haraam relationship; should he marry his former girlfriend if she becomes Muslim?

I have been a muslim all my life and I have done some terrible things in my that has cause some great distress in my life. I have been with a Christian girl for 2 years now but recently seperated from her because I have came closer to islam. She understands that we should not be together right now. I know that I wasn't a good muslim before but now I want to do anything to be for given for what I did. We have done some sexual things like fondling, manual pleasure, and oral sex on one occasion. I know that these are grievous things and I beg Allah for forgiveness and regret what I have done. This was a result in my ignorance and I have repented. I now pray 5 times a day, go to the mosc, and read Quran. I never want to go back to the bad things that I have done. I feel that I should marry a muslima from Egypt and change my life but I also feel that I should at least show the Christian girl islam in hopes that she will convert. My questions are: Should I marry her if she converts? Or should I find a muslima and start a new life? Will I be forgiven for what I have done?.

Praise be to Allaah.
Firstly:

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.

See also: 624, 13990, 34905 and 22912

Secondly:

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.

Thirdly:

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.

See 22303 and 2527.

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.

See 20227 and 8391.

And Allaah knows best.