Grandma's Posthumous Letter

While sorting through the belongings of my late grandmother, I found a locked wooden box. Inside there was only a yellowed letter, with just one sentence written on it: "At this time tomorrow, you will receive another identical letter. Do not open it, do not keep it." I paid no mind to it, and left the letter on the corner of my desk. At the same time the next day, I truly did receive an identical letter — the handwriting, the paper, the content, even the creases were exactly the same. For the next five days, at the same time every day, I received an identical letter. The pile of letters on the corner of my desk grew taller and taller. I never opened any of the letters, nor did I throw any of them away. On the seventh day, I did not receive a new letter. Instead, I found that all the letters on the corner of my desk had disappeared. I walked to my grandmother's old wardrobe, and the moment I opened the door, I collapsed to the ground and lost all vital signs. Please deduce: the source of the letters, the meaning of the words on the letter, the cause of the letter cycle, the reason the letters disappeared, the secret in the wardrobe, and the truth of my death (six core layers of truth, no conventional tropes).