The Vanished Reflection

When I woke up, I was lying in a completely sealed room. The room only has one glassless window (impossible to enter or exit through), one bed, one full-length mirror, and a note on the bedside that has only one line written on it: "Everything you see is not real; what you cannot find is the key". I got up and walked to the full-length mirror, and saw my own face clearly, exactly the same as I remembered it. I touched the mirror, it was cold and hard, and I confirmed it was a real mirror. Over the next three days, I stared into the mirror for a long time every day, trying to find a clue to leave, but I found nothing. Only my reflection in the mirror, doing exactly the same movements as me in perfect sync. On the fourth morning, I walked to the mirror again, and suddenly found that my figure was gone from the mirror. I did not panic, but instead smiled in relief, then slowly closed my eyes, and never opened them again. Please reason out: the purpose of the room, the true meaning of the note, the secret of the mirror, the reason why I can't see myself in the mirror, the reason I felt relieved, the truth of my death (six layers of core truth, closely linked, all indispensable, no irrelevant settings).