All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
You are obliged to take the initiative to repent and cut any relationship that you have with this girl as such a relationship is a means that leads to committing what causes the anger of Allaah. Allaah Says (what means): {And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.}[Quran 17:32] The scholars said: “Allaah cautioned us by the word ‘approach’ as a way of attracting our attention to be careful of the means that leads to it (i.e. to unlawful sexual intercourse).” For more benefit, please refer to Fataawa 81356 and 86527.
On the other hand, you should take the initiative to marry, as this is the best way to protect yourself from Zina (fornication or adultery). If a person fears he will commit Zina, marriage becomes an obligation for him. For more benefit on the ruling on marriage, please refer to Fataawa 81853 and 84026.
As for your parents, you should try to convince them to accept that you marry. If they accept, then this is what is required, and if they insist on refusing, then you are not obliged to obey them because it is not an obligation to obey the parents in matters that cause harm to the son. For more benefit on the limits of obedience to parents, please refer to Fatwa 84942. Moreover, you are not obliged to inform your parents about your marriage.
As regards forging documents, then this is forbidden and the Prophet considered it as one of the grave major sins as in the Hadeeth reported by Al-Bukhari and Muslim so it is not permissible to forge documents except in case of necessity and there is no necessity in your case, because it is possible to carry out a fake contract just to carry out the procedures of the court while this contract has no effect. Rather, what is effective is the first contract.
The following matters should be mentioned:
1- According to the most preponderant opinion, the guardian is a condition for the validity of marriage, so the Muslim guardian of this girl is the one who has the right to carry out the marriage contract. If she has no Muslim guardian, then it is the Muslim judge who should marry her off or any one who acts on his behalf in your country. For more benefit, please refer to Fatwa 83629.
2- You are obliged to expiate for every oath that you broke and returned to the sin after it; for more benefit, please refer to Fatwa 82135.
Allaah Knows best.