How does a young man who committed adultery with a married woman repent of his sin, what should he do to stay away from indecencies, and will the woman's supplication against him be accepted if he severs his relationship with her?

What you have done is a great sin and transgression, and what has befallen you is some of the consequences of disobedience. Therefore, you must hasten to sincere repentance, regret the sin, and resolve not to return to it. Begin by cutting off your relationship with the girl and every forbidden relationship, and block the paths leading to evil. If you repent sincerely, then trust in Allah's forgiveness, and seek help in repentance through remembrance (dhikr) and supplication (dua), the company of the righteous, and increasing good deeds; for good deeds erase bad deeds.