I've fallen because I've been on the precipice
A precipice is a very high, or steep rockface. He is implying that he’s fallen into derealization off the “high” cliff due to actually getting high off drugs, which is a possible trigger to this mental illness.
“Cannabis, psychedelics, dissociatives, antidepressants, caffeine, nitrous oxide, albuterol, and nicotine can all produce feelings resembling derealization, particularly when taken in excess. It can also result from alcohol withdrawal or benzodiazepine withdrawal.”
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