This is such a difficult question -- I honestly wish I knew the answer outright and could give it to you.
I don't think I can do that much, but what I can do is offer some theories and context.
(Note: I wrote at length about this experience, and my general experiences with both psychedelics + MDMA here: https://news.ycombinator.com/item?id=22991744)
I'll write the following under the assumption that you/the reader don't have experience with psychedelics. There are some things you can put into words for those unfamiliar, and a great many other things you cannot.
---
Psychedelics are a fundamentally neutral substance. What I mean by that is, that they and the experience, are neither good nor bad. As hippy as it sounds, "they are what you make of them".
This quality, if you ask me, boils down to one property: Psychedelics are enhancers. If you feel beautiful and happy and lively, you feel REALLY beautiful, happy, and lively.
If you start to become frightened/anxious of something, or are in a frightening situation, you become REALLY frightened.
And this becomes further amplified by a second property of psychedelics:
Rather than the usual business of "Some external occurrence or sensation happened, and here is my one thought/reaction to it." IE "Wow, that statement was rude, what a jerk." Or "Man, that dark alleyway looks kind of scary, I think I'll not go that way."
Your subconscious and free-association become stronger respective of dose, to the point where they can become runaway.
No longer is the equation "1 external action = 1 internal processing response", but some small experience can set off a chain reaction of internal association tumbling down a bunch of different paths.
Dark alleyway -> Oh that's scary -> That's the sort of place monsters would be -> Monsters, oh those are terrifying -> Murder is terrifying -> I don't want to be murdered -> There's probably a monster in that dark alleyway -> Dark alleyway, that's so terrifying -> You know what's probably in there, a monster? -> A monster would murder me -> I don't want to die -> Die.. death...
It becomes a looping, inescapable schizophrenic sort of experience where you've also lost enough short-term memory to forget you've been going in loops like this for what feels like years.
And because things are so ENHANCED, the amount of screaming terror, dread, etc you can feel is beyond any thing you can comprehend. Time is slowed to a crawl, this can continue for eternity.
Now, if that all sounds awful -- just imagine this whole scenario, but filled with the most intense happiness and love you can imagine. That's what MOST experiences are like, generally.
---
What happened that night is I started getting anxious and fell into a dark place in my mind and got stuck there for an eternity instead of a happy place, to put it shortly.
I also won't pretend I probably didn't have a lot of subconscious trauma from a really messed up childhood and likely other mental issues/insecurities.