If one repents from a sin, then returns to it, was his repentance not sincere?

Question:
He says: A person repented to Allah from committing sins and avoided the causes that lead to them, but whenever the causes of fitnah (trial, discord or temptation) are presented to him or he is exposed to them, he is weak against them and falls into what is forbidden. Therefore, he strives with all his might to avoid these causes. He asks: Is his repentance not sincere? The one whose repentance is sincere, no matter what fitnah (trial, discord or temptation) he encounters, is strong against it. Yes.

Answer:
His repentance, if its conditions are met, is valid, if Allah wills. This means that he abandons the sin he repented from, resolves not to return to it, and feels remorse for the disobedience he committed. If these conditions are met, the repentance is valid. If the repentance is regarding the right of a created being, a fourth condition is added to these: he must return the right to its owner and seek forgiveness from the person involved. If these conditions are met, the repentance is valid and acceptable to Allah (Glorified and Exalted is He), as Allah promised. The fact that a person fears for himself from fitnah (trial, discord or temptation) when its causes are presented to him is evidence of faith and that the person does not declare himself pure, because no one among people is safe from fitnah (trial, discord or temptation) for themselves, as humans are weak. And Allah the Exalted said: . However, it is obligatory for a person to anticipate the causes of fitnah (trial, discord or temptation) and stay away from them in order to be safe from their evil and falling into sin again. So he must avoid the causes of sins and the causes of fitnah (trial, discord or temptation), and stay away from them, preferring safety from them. Yes.