What is the reason for atheism despite memorizing and recalling verses, and what is the impact of that on one's psychological state and worship?

The straying of one who has gone astray or the disbelief of one who has disbelieved is not an argument against the steadfastness of one who has believed and been guided. Allah has mentioned in the Quran the state of those who went astray after guidance and disbelieved after faith, clarifying their recompense and the reason for their falling into it.

As for the sister who is asking, if she does not find a specific doubt or problem that leads her to suspicion, then what she is complaining about may merely be whispers from Satan and thoughts from the inciting self. She should seek refuge in Allah sincerely, and frequently remember Him and recite His book, as these are the greatest means of guidance. She should also strive to keep good, sincere company.