He wants to marry a girl with whom he had a relationship
Question

                I hope you can help me as I am in a deep trouble. I knew some girl who works far from where her family lives. For two years we had a loving relationship, we were meeting and committing Zina (adultery), we agreed to marry because I can not forget her, and she can not forget me either. Since she knew me she became religious and changed a lot. Allah knows how much I love her. Do you advise me to marry her? I am suffering.
Praise be to Allah.
    Firstly: 

    Before answering your question, we must remind you that it is 
    obligatory to repent and regret what you have done with this woman, because 
    you have fallen into several major sins, the most serious of which is zina 
    (fornication) which is clearly forbidden in the Qur'aan and Sunnah, and the 
    scholars are unanimously agreed that it is haraam, and wise people are 
    unanimously agreed that it is abhorrent and evil. 

    Allaah says (interpretation of the meaning): 

    “And come not near to unlawful sex. Verily, it is a 
    Faahishah (i.e. anything that transgresses its limits: a great sin), and an 
    evil way that leads one to hell unless Allaah forgives him)”

    [al-Isra’ 17:32]

    And the Prophet  (peace and blessings of Allaah be upon 
    him) said: “No adulterer is a believer at the time when he is committing 
    adultery.” Narrated by al-Bukhaari (2475) and Muslim (57).  

    There is a severe punishment for zina in al-Barzakh, before 
    the punishment in the Hereafter. In the famous hadeeth of Samurah ibn Jundub 
    (may Allaah be pleased with him) about the dream, it says:  

    “… then we [i.e., the Prophet  (peace and blessings of 
    Allaah be upon him) and Jibreel and Mikaa’eel] proceeded and came to 
    something like a tannoor (a kind of oven), in which there were clamouring 
    voices.” He [the Prophet (peace and blessings of Allaah be upon him)] said: 
    “We looked into it and there we saw naked men and women. Flames were coming 
    to them from the bottom of it, and when the flames reached them, they made 
    uproar. I said to them [i.e., the two angels who were accompanying him], 
    ‘Who are these?’ … They said, ‘We will tell you. As for the naked men and 
    women in the structure that resembled a tannoor oven, they are the 
    adulterers and adulteresses.’” 

    Narrated by al-Bukhaari (6640). 

    Allaah has ordained the hadd punishment for zina. He says 
    concerning the hadd punishment for an unmarried person (interpretation of 
    the meaning): 

    “The fornicatress and the fornicator, flog each of them 
    with a hundred stripes. Let not pity withhold you in their case, in a 
    punishment prescribed by Allaah, if you believe in Allaah and the Last Day. 
    And let a party of the believers witness their punishment”

    [al-Noor 24:2]

    As for the person who has previously been married, the hadd 
    punishment is execution, In the hadeeth narrated by Imam Muslim in his 
    Saheeh (3199) it is narrated that the Prophet  (peace and blessings 
    of Allaah be upon him) said: “For a previously-married person with a 
    previously-married person, [the punishment is] one hundred lashes and 
    stoning.” 

    What we have said to you applies to the woman too, and she 
    should realize that her sin is even worse, but because, as you say, she has 
    become righteous, we hope that her repentance is sincere and that Allaah 
    will forgive her by His grace and kindness. 

    Secondly: 

    You should note that if you have not both repented from the 
    sin of zina, then it is not permissible for you to marry her, because Allaah 
    has forbidden the zaani and zaaniyah to marry unless they both repent. 
    Allaah says (interpretation of the meaning): 

    “The adulterer — fornicator marries not but an adulteress 
    — fornicatress or a Mushrikah; and the adulteress –fornicatress, none 
    marries her except an adulterer — fornicater or a Mushrik [and that means 
    that the man who agrees to marry (have a sexual relation with) a Mushrikah 
    (female polytheist, pagan or idolatress) or a prostitute, then surely, he is 
    either an adulterer — fornicator, or a Mushrik (polytheist, pagan or 
    idolater). And the woman who agrees to marry (have a sexual relation with) a 
    Mushrik (polytheist, pagan or idolater) or an adulterer — fornicator, then 
    she is either a prostitute or a Mushrikah (female polytheist, pagan, or 
    idolatress)]. Such a thing is forbidden to the believers (of Islamic 
    Monotheism)”

    [al-Noor 24:3]

    Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allaah have mercy on 
    him) said: 

    This clearly points to the abhorrent nature of zina, and that 
    is tarnishes the honour of the one who does it in a way that other sins do 
    not. Allaah tells us that no woman would marry a zaani but a woman who is 
    also a zaaniyah, who is like him, or a mushrik woman who associates others 
    with Allaah and does not believe in the Resurrection or in reward and 
    punishment (in the Hereafter), and who does not adhere to the commands of 
    Allaah. And similarly, no one would marry a zaaniyah except a zaani or a 
    mushrik. “Such a thing is forbidden to the believers” means, it is 
    haraam for them to marry zaanis or zaaniyahs. 

    What the verse means is that if a person wants to marry the 
    man or woman who commits zina and has not repented from that, despite the 
    fact that Allaah has prohibited that, then he is either not adhering to the 
    ruling of Allaah and His Messenger, in which case he cannot be anything but 
    a mushrik, or he is adhering to the ruling of Allaah and His Messenger but 
    he agrees to this marriage despite knowing that this person has committed 
    zina, in which case this marriage is also zina, and he is an immoral zaani. 
    If he truly believed in Allaah, he would not do that. This clearly indicates 
    that it is haraam to marry a zaaniyah unless she repents, or to marry a 
    zaani unless he repents, because marriage is the strongest type of 
    companionship, and Allaah says (interpretation of the meaning): “Assemble 
    those who did wrong, together with their companions” [al-Saaffaat 37:22]. 
    Allaah has forbidden that because of what it involves of great evil, and 
    lack of protective jealousy, and attribution of children who are not his to 
    the husband, and the zaani failing to keep her chaste because he is 
    distracted elsewhere, any one of which is sufficient reason for the 
    prohibition. End quote. 

    Tafseer al-Sa’di (p. 561). 

    The scholars of the Standing Committee were asked: 

    A man committed zina with a virgin and wants to marry her. Is 
    it permissible for him to do that? 

    They replied: 

    If the matter is as described, each of them must repent to 
    Allaah and give up this sin, and regret what has happened of immoral 
    actions, and resolve not to do it again, and do a lot of good deeds, in the 
    hope that Allaah will accept their repentance and turn their bad deeds into 
    good. Allaah says (interpretation of the meaning): 
“And those who invoke 
    not any other ilaah (god) along with Allaah, nor kill such person as Allaah 
    has forbidden, except for just cause, nor commit illegal sexual intercourse 
    ___ and whoever does this shall receive the punishment.
69. The torment will be 
    doubled to him on the Day of Resurrection, and he will abide therein in 
    disgrace;
70. Except those who 
    repent and believe (in Islamic Monotheism), and do righteous deeds; for 
    those, Allaah will change their sins into good deeds, and Allaah is 
    Oft‑Forgiving, Most Merciful
71. And whosoever 
    repents and does righteous good deeds; then verily, he repents towards 
    Allaah with true repentance”
[al-Furqaan 25:68-70]

    If he wants to marry her, then he must wait for one menstrual 
    cycle to establish whether her womb is empty before doing the marriage 
    contract with her. If it turns out that she is pregnant, then it is not 
    permissible for him to do the marriage contract with her until after she 
    gives birth, in accordance with the hadeeth in which the Prophet  (peace 
    and blessings of Allaah be upon him) forbade a man to water the crop of 
    another with his own water. End quote. 

    Fataawa Islamiyyah (3/247). 

    So repent to Allaah and set your affairs straight, and do a 
    lot of good deeds, and after that it will be permissible for you to get 
    married. We ask Allaah to accept your repentance and to forgive you, by His 
    grace and mercy. 

    See also question no. 85335. 

    And Allaah knows best.