Praise be to Allaah.
It seems that you are confusing matters of the heart with matters of aqeedah and religion, because you state in your question that you were emotionally devastated when this woman left you and went back to her kaafir boyfriend. You need to keep matters of belief and religion separate from your emotions and personal desires. Adhere to the limits set by Allaah, and follow His rules, one of which is the aayah (interpretation of the meaning): Let no man guilty of fornication or adultery marry any but a woman similarly guilty, or an Unbeliever; nor let any but such a man or an Unbeliever marry such a woman; to the Believers such a thing is forbidden. [al-Noor 24:3] It is not permitted to marry such a woman unless she repents, and this woman has not repented, she has gone back to committing haraam deed with a kaafir. So do not waste your time feeling regret about her. Ask Allaah to bless you with a chaste, believing wife, one who fasts and prays and devoutly adheres to the limits set by Allaah. This is the kind of woman you should long for, not this impure woman whose interest in Islam was only a temporary, emotional affair. I also advise you to adhere to the shareeah when calling non-Muslims to Islam, and not to treat the matter of speaking to non-mahram women (women to whom you are not related) lightly. You should not start any kind of forbidden contact or relationship with them in the name of dawah. The Muslim man should use indirect means of communication when calling non-Muslim women to Islam, such as giving them books or tapes, or communicating with them via e-mail, etc., within the bounds of what is right and proper. We ask Allaah to guide us all and make us content with what is halaal so that we will have no desire for anything that is haraam. May Allaah bless our Prophet Muhammad.