Is what transpired between the questioner and the young man considered fornication, and how does she know that God has forgiven her sin, and how does she remain steadfast in cutting off the relationship with the young man she loves?

What transpired between you and this person is not the type of adultery that necessitates the prescribed legal punishment (hadd), but it is of the general kind of zina (illicit sexual activity) mentioned by the Prophet, peace and blessings be upon him: "Indeed, the son of Adam has been allotted his share of zina, which he will inevitably attain. The zina of the eyes is looking, the zina of the ears is listening, the zina of the tongue is speaking, the zina of the hand is striking, the zina of the leg is stepping, and the heart yearns and wishes, and the private parts affirm or deny that." You must repent to Allah Almighty by abandoning the sin, regretting it, and resolving never to return to it. You should also take the means that aid in repentance, such as cutting off contact with anyone who facilitates sin, choosing righteous company, staying away from anything that arouses desires, and increasing in supplication and voluntary prayers. If repentance fulfills its conditions, its acceptance is hoped for, for Allah Almighty is Oft-Forgiving, Most Merciful.