Why does God afflict the questioner with whispers (waswas) of illness and death, despite her diligently observing the obligatory prayers, remembrances (adhkar), and supplications (du'as) such as "O Allah, I seek refuge in You from anxiety and grief" and "O Allah, make us servants of beneficence"?

You should ignore misgivings and not pay attention to them, submitting to Allah's decree and being content with His قضائه (divine judgment), recognizing that everything Allah decrees is good, merciful, and wise. No one can fully fulfill Allah's right, and affliction expiates sins. Allah Almighty says: "And whatever strikes you of disaster - it is because of what your [own] hands have earned, and He pardons much." Allah has wisdom and interests in trials that a servant may be unaware of, and a believer worships through patience during affliction and gratitude during prosperity. Patience in affliction and contentment with divine judgment have good consequences, and the patient will be given their reward without reckoning. Do not despair over Allah's decrees; rather, strive in supplication and drawing closer to Him.