How do we reconcile the certainty of prayer being answered with the fear of it not being answered due to sins?

Scholars have differed in interpreting the certitude (al-yaqīn) mentioned in the Hadith: "Supplicate to Allah while you are certain of a response." Its meaning could either be: be in a state that makes you deserving of a response, or believe that Allah Almighty will answer your supplication. There is no contradiction between being certain of a response and fearing that the supplication might be rejected due to sins, because Allah has promised to answer, but He also clarified that some sins prevent the response, as stated in the Hadith: "A man lengthens his journey, disheveled and dusty, he stretches his hands towards the sky, [saying]: 'O Lord, O Lord,' while his food is unlawful, his drink is unlawful, his clothing is unlawful, and he has been nourished with unlawful things, so how can he be answered?" Thus, a Muslim supplicates with certainty, due to his good opinion of Allah, and fears that his supplication might be rejected due to his poor opinion of himself and his shortcomings.