How can one help a young man (23 years old) who wishes to repent and return to Allah after committing many repeated sins and transgressions, experiencing previous relapses, and feeling the sweetness of obedience despite having no one to help him remain steadfast?

Your feeling of guilt is a good sign, and thank Allah that you are still alive to repent. Hasten to sincere repentance from sins, especially disobedience to parents and adultery, and remember that Allah fulfills His promise to those who are sincere with Him. Review the conditions for repentance and the means to remain steadfast upon it. If you are in an environment of temptation, emigrate from it to a righteous environment where you can preserve your religion.