What is the ruling of Islamic law regarding marriage to a girl whom I promised to marry, and something less than fornication occurred between us, and she is now ill and accuses me of injustice after I ended the relationship with her?

What is obligatory upon you is to repent to Allah Almighty for what transpired between you and this girl. This girl wronged herself by what she did, and you were neither an oppressor to her nor did you coerce her. Do not demand to marry her, and you are not obligated to fulfill your promise to her of marriage. However, if she repents, turns back [to Allah], and improves her conduct, then there is no harm in marrying her. If you do not marry her, then part ways with her and sever all ties. As for Allah having forgiven you the unseen, if you mean that He spared you from falling into the great tribulation of immorality, then this meaning is correct.