Is the questioner considered to have erred in ending his relationship with a girl after transgressions occurred between them, given his feeling of guilt for having provoked her and not advised her, and how can he rectify his mistake, overcome doubt, and make decisions with confidence?

The feeling of wrongdoing and guilt is the beginning of the path to repentance. First: sever the unlawful relationship with this girl and repent to Allah. Second: do not judge the girl as unchaste merely because of this slip; rather, look at her general condition. If her general condition is righteousness, then there is no harm in marrying her. Third: keep religion in mind when choosing a life partner, so strive to marry a religious woman who will please you, obey you, and protect you. Repentance and Istikhara (prayer for guidance) before undertaking an action will remove doubts from you. There is no harm for you in marrying this girl if you know from her condition that she is not persistent in sin, and that what occurred from her was merely a slip.