How can I reconcile memorizing the Quran and repeatedly falling into secret sins, along with immediate repentance, given my fear that the Quran will be a testament against me on the Day of Judgment?

You must continue to memorize the Qur’an and act upon it, and repent after every sin, for he who repents from sin is like one who has no sin. Among the most important reasons for steadfastness is to adhere to obedience and increase therein, for Allah Almighty says: "And those who strive for Us - We will surely guide them to Our ways." (Al-Ankabut: 69). You must increase in good deeds along with sincere repentance, for Allah Almighty says: "Indeed, good deeds do away with evil deeds." (Hud: 114), and do not combine sin with the abandonment of obedience.