How can a woman protect herself from returning to sin after her repentance and turning back to Allah (the Most High)?

You made a mistake by speaking with this young man, going out with him, and being alone with him, for being alone with a non-mahram woman leads to indecency unless Allah grants protection, because Satan is the third of them. You must initiate sincere repentance, increase in righteous deeds, distance yourself from the causes of temptation, and supplicate frequently for Allah to keep you firm upon the truth.