Is God capable of annihilating everything, such that only God remains as He was before creation? And is the meaning of creation ex nihilo a demonstration of God's power? And does one who disbelieves in God's ability to annihilate disbelieve, knowing that the questioner suffers from obsessive thoughts about these issues?

Allah Almighty is capable of all things. Just as He brought creation into existence, He is capable of bringing it to an end. Everything Allah created, He is capable of annihilating. However, despite Allah the Almighty's ability to annihilate every creature, He will not do so. For among His creations, there are those He created for permanence, such as Paradise, Hell, the Throne (Al-Arsh), the Footstool (Al-Kursī), the Preserved Tablet (Al-Lawh), the Pen (Al-Qalam), and the Trumpet (As-Sūr). No one has claimed the annihilation of all creatures except a group of innovative theologians (Ahl al-Kalam). This is a false statement that contradicts the Book of Allah, the Sunnah of His Messenger, and the consensus of the early generations (Salaf) of the Ummah and its imams.

The best remedy for obsessive thoughts is to seek refuge in Allah, to stop engaging with them, and not to yield to them. Hating these thoughts is a sign of faith, and one is rewarded for struggling against them. It is also advisable to consult a specialized psychiatrist, as obsessive-compulsive disorder (OCD) has medical and cognitive-behavioral treatments. Combining both faith-based and medical treatments is more effective and curative, by Allah's will.