Then you are a normal believer being changed slowly, not a fraud losing your salvation. Repentance is a change of mind about where you put your trust. Growth follows, but it is God's work in you and it takes time.
The Grace Answer
The pain in this question is real, and it usually comes from a sincere heart, not a careless one. Careless people do not lie awake worried that their repentance did not stick. So before anything else: the very fact that this troubles you is evidence the Spirit is at work in you, not evidence that He has given up.
Part of the problem is a misunderstanding of the word repent. We tend to hear it as stop sinning successfully, or else, as if repentance were a performance you either pull off or fail. But the word means a change of mind. Under grace, the deepest repentance is not primarily about behavior management. It is turning your trust away from your own effort and back toward Christ. You keep changing your mind about who saves you. That is repentance, and it can be genuine long before your behavior fully catches up.
Change is real, but it is gradual
Scripture never promises instant perfection at conversion. It promises a process: “He who has begun a good work in you will complete it.” Notice who is doing the completing. Not you, gritting your teeth. Him, finishing what He started. Growth in the New Covenant works from the inside out and from identity to behavior, which means it is often slower than we want and surer than we fear.
So what do you do with a sin you keep returning to? Not panic, and not pretend. You keep bringing it into the light, you keep trusting the One who already forgave it, and you let Him do the deep work that white-knuckled willpower never could. A struggle you have not yet won is not the same as a salvation you have lost. One is the ordinary shape of being discipled. The other never happens to a child God has promised to keep.