For marriage in Islam to be valid, it must be done not as a secret affair but as a public event with witnesses and the permission of the guardian, the payment of the mahr, as well as publicizing the relationship which is known as ishhar. The purpose of announcing marriage is to clear all doubts or suspicion about the relationship of the couple. That is why the Prophet (peace and blessings be upon him) is reported to have ordered his companions saying: “Publicize the marriage.” This is being done so that people do not raise questions about this relation.
The European Council for Fatwa and Research, states the following: The contract in the way the enquirer has described in the question is invalid because it lacks the Shari`ah (legal) conditions. It was made without a guardian or witnesses, and none of the followed imams has validated a contract of this type. The Hanifis and those who share their opinion, who do not stipulate the guardian, claiming that a Muslim woman can marry herself, stipulate the necessity of present witnesses. Those who do not stipulate the necessity of witnesses to the contract stipulate it before the consummation, which is the view of the Malikis who stipulate that the contract should be made in the presence of the legal guardian of the Muslim woman, and in case he is not available because of his death or due to him being legally unqualified (because he is not a Muslim), the guardianship is transferred to one of the other Muslims present.
Therefore, you have to stop indulging in sexual relationship till you make a new contract at the presence of Muslim witnesses and the guardian of the woman if available. And if he is not available it will be enough for her to authorize one of the Muslims to marry her to you. As for the sexual relationship practiced before this ruling, one must ask Allah’s forgiveness for it.