Would I be unjust to that girl if I left her after all these years and the forbidden acts that transpired between us?

Lenience with a foreign woman leads to fitna and corruption, and what is obligatory is repentance for what has passed. The questioner must complete his marriage to the girl if she becomes righteous, and work to reconcile relations between the families through respected figures. There is no need to rush into divorce as long as the wife is not insistent on committing sins, for family problems do not justify divorce as long as the spouses are in agreement. Divorce is permissible when there is a need for it, detestable without a reason, and may be forbidden. The questioner is not oppressing her, for what happened was with her consent.