What should I do to absolve myself from the sin of causing a young man to stray, with whom I had a relationship before I became committed, knowing that "whoever guides to a sin will have a reward like that of its doer"?

The sister who asked the question should be a cause for the guidance of the young man whom she caused to deviate. This can be achieved by encouraging him to repent and return to Allah Almighty in an appropriate manner, by having righteous people, through her maharim or the husbands of her friends, influence him. She must be extremely careful to avoid any direct contact with him. If she exerts her utmost effort in this, she will have fulfilled her obligation, and she will be rewarded for his guidance if he repents. Otherwise, there is no blame on her, because guidance is in the hands of Allah Almighty, as confirmed by His saying: "Indeed, you do not guide whom you like, but Allah guides whom He wills. And He knows best of the guided."