How can one repent from unlawfully touching and intimately approaching a sister without committing adultery, and what is the way to avoid suicidal thoughts after repentance?

Allah has forbidden the preliminaries of zina (fornication/adultery), and the sin intensifies with maharem (unmarriageable kin). To be saved, you must do the following: 1. Remain steadfast in repentance: Allah is Forgiving and Merciful; He accepts repentance. So, repent to Him and increase in righteous deeds, for "indeed, good deeds do away with evil deeds." 2. Fasting: Increase your fasting, as it curbs desire, for it is a "shield." 3. Stay away from your sister: Keep away from your sister and do not be alone with her, especially if you still feel an inclination towards her. 4. Marriage: Strive to marry to protect yourself from the forbidden. 5. Manners of seeking permission: Maintain the manners of seeking permission when entering upon anyone.

Regret is a sign of repentance, and suicide is not a solution.