What should I do about the difficult intrusive thoughts regarding matters of faith, which make me fear disbelief and affect my acts of worship, and should I ignore them despite the thoughts that pursue me?

The cure for obsessive thoughts (waswasah) is to completely disregard them and not dwell on them. Obsessive thoughts come from Satan, and their cure lies in seeking refuge (with Allah), prayer, and abundant remembrance (dhikr). Ibn Hajar al-Haytami clarified that the effective remedy for obsessive thoughts is to sufficiently and entirely disregard them. He stated that whoever pays no attention to them will find them disappear after a short while, but whoever listens to them will be led into the realm of the insane. Al-'Izz ibn 'Abd al-Salam mentioned that obsessive thoughts are satanic whispers, and fighting them is a reward for the struggler. Obsessive thoughts only take hold of the ignorant. As for one who possesses true knowledge and intellect, they will not deviate from adherence (to Islamic principles). It is recommended for someone afflicted with obsessive thoughts concerning ablution (wudu') or prayer to say: "There is no god but Allah," for when Satan hears remembrance, he retreats. The person suffering from obsessive-compulsive disorder (OCD) is not held accountable for their obsessive thoughts, especially if they fear disbelief (kufr). The Prophet, peace and blessings be upon him, said regarding obsessive thoughts: "That is clear faith," meaning that deeming such thoughts to be grave is a sign of clear faith. This applies to one whose faith is complete and from whom doubt and suspicion have been dispelled. Alternatively, it means that Satan whispers to those whom he has despaired of misleading.