What is the ruling on someone who sinned against another person and repented, but cannot ask for their forgiveness? Is their repentance accepted, and what is required of them to do?

Congratulations on your guidance and repentance from the sinful relationship. It is not permissible, according to Islamic law, for a woman to be in an emotional relationship with a non-mahram (definition) man. If the alleged injustice is merely the termination of the relationship, then this is not an injustice; rather, it is the epitome of wisdom and justice. Life must be resumed, and effort should be directed towards what benefits one in matters of religion and worldly affairs.