I've had that exact feeling of the 'dreadful alternative' of immortality under the influence of (in retrospect) way too much THC. Also, much of the perceived 'peace' and 'tranquility' some people describe feeling from a perception of being 'connected to everything' manifested the exact opposite feelings in me. All of a sudden I felt completely alone. I do not want to be 'one' with everything because that means there is nothing beyond me.

Perhaps a more positive experience might be one in which you still feel distinct but nevertheless intimately connected with everything. I've done some psilocybin, but never reached anything close to 'ego-death.' I am curious to hear other people's thoughts on how these ego-less experiences make one at peace, and not in complete existential horror (I suppose if there is no ego, there is nothing to 'feel' the horror?).