What should I do regarding the prayers and fasting that I missed before becoming religiously committed, and what must I do to rectify what I ruined with the girl with whom I had a previous relationship, knowing that she is not yet married and is still not religiously committed?

Repentance from sins has valid conditions. As for making up missed acts of worship like prayer and fasting, there are two scholarly opinions: The majority view necessitates making them up, while another view does not require it, especially if one considers the prayer-leaver a disbeliever, because in that case, repentance is considered an embracing of Islam that nullifies what came before it. Authentic evidence indicates the disbelief of the prayer-leaver, whether he abandoned it out of laziness or denial. Obliging the repentant person to make up these acts might make repentance difficult, and he should instead increase in righteous deeds. The Permanent Committee has issued a fatwa stating that there is no obligation to make up prayers and fasts for one who repents from intentionally abandoning them, citing the Prophet's saying (peace be upon him): "Islam demolishes what came before it, and repentance demolishes what came before it." It is also necessary to cut off all ties with the girl to avoid a relapse into disobedience.