What is the ruling on the psychological fear and anxiety that come with nightfall, and on the whispers that cast doubt on the virtue of prayer after one has committed to it, and what is the ruling on intense fear of the grave and the dead, to the extent of being unable to visit cemeteries?

The psychological distress and obsessive thoughts you are experiencing are from Satan, intended to cut you off from returning to Allah. Prayer is one of the greatest means of attaining tranquility, and when a Muslim prostrates to Allah, Satan weeps. You must abandon these obsessive thoughts, increase your supplications (du'a), and engage in remembrance (dhikr). Do not ever entertain the thought of abandoning prayer, as it is one of the pillars of Islam, and the faith of one who abandons it is questionable. Your fear of the dead emerging from graves is also from the whispers of Satan. Visiting graves has been legislated for drawing lessons and remembering the Hereafter. Do not visit them alone; rather, accompany righteous people so that you do not succumb to obsessive thoughts.