How can one treat the fluctuation of faith between obedience and disobedience, and the recurring return to sins after repentance, especially with the presence of the root of faith, and the fear of not being steadfast on the Day of Resurrection?

The solution lies in a sincere determination to act, for Satan wants to draw you into disobedience so you become accustomed to it. If you fall into sin, hasten to sincere repentance and do not surrender to Satan's cunning. Sense the danger of sins and the fear of an evil end, and remember that death may come suddenly. Adhere to remembrance (dhikr), supplication (du'a), and seeking forgiveness (istighfar). If you commit a sin, follow it with extra good deeds such as recitation of the Quran, prayer, and fasting. Accompany the righteous and do not abandon their company.