How can one atone for secret sins and avoid falling into them in the future, while fearing a bad end and drifting away from God?

Expiation for sins committed in private is achieved through sincere repentance that meets its conditions, which are: abandoning the sin, resolving not to return to it, and regretting having committed it. If the sin is repeated, one must renew repentance and not despair of it. Among the things that aid in repentance are: recalling God's full awareness and encompassing knowledge of His servant, contemplating the horrors of the Day of Judgment, Paradise, and Hell, persistently supplicating for steadfastness, adhering to the remembrance and constant awareness of God, keeping company with the righteous, and frequently reading about the consequences of sins.