Is what the questioner is doing — singling out a Christian girl, with whom he had an illicit relationship, for prayers for guidance, and giving charity and fasting with the intention that Allah unite them in a lawful marriage after his repentance — permissible in Sharia or forbidden?

Supplicate for the acceptance of repentance and for an increase in good deeds. We also recommend the supplication reported from Anas (may Allah be pleased with him): "O Turner of hearts, keep my heart steadfast upon Your religion." There is no harm in supplicating that Allah facilitate for that girl to embrace Islam and for you to marry her, provided she has not married someone else. You must avoid regretting not marrying her if she was not chaste, as it is impermissible to marry a non-Muslim woman who is not chaste. We advise seeking a righteous Muslim wife. We also draw attention to the fact that residing in lands of disbelief is a cause of many trials, and it is better for one who has no necessity there to emigrate to a Muslim country.