What is the ruling on the thoughts that constantly occur to a person and make him feel that he has not achieved the meaning of "there is no god but Allah," and affect his interaction with the Qur'an, and has everything gone from him, and what should he do in this situation?

These thoughts are whispers (waswas), and their cure is to ignore them. Occupy your time with acts of obedience, acquire beneficial knowledge, engage in productive work, beware of solitude, and seek righteous companionship. Aspire to memorize the Qur'an and Sunnah, and frequently read books of the Sirah (Prophet's biography) and those on spiritual refinement (Raqaiq). If these thoughts come to you, seek refuge with Allah from Satan, and busy yourself with remembrance (dhikr) and recitation (of the Qur'an).