Several weeks ago, I had an episode of derealization and depersonalization, lasting with various severity for one or two weeks. During this time, I also had a tooth and inner ear infection that required a root canal. The link between this infection and my psychological state was formed in various ways: pain, sleeplessness, anxiety and perhaps a reaction to several drugs given to me tipped me over into an unusual, and unpleasant state. There was also some unpleasant anxiety with my housing situation, which is somewhat relevant to the feeling.
There are many different aspects of derealization and depersonalization, and I can hardly claim to be a clinical expert based on one or two weeks of feeling it. However, I do have one good way to explain the feeling.
Like most people, I've moved many times in my life. I know the feeling of stepping into a spotlessly clean and featureless apartment or house, and how that feeling of emptiness and contextlessness rapidly fades as memories and associations and physical cruft collect, turning an arbitrary space into something that feels like a home. And things around me become comfortable and familiar, and I have dozens or hundreds of little things to remind me of who and where I am. But then I've also left a house, and seen the process reversed, seen something that meant so much to me turn back into an anonymous structure. And sometimes, inbetween times, I will have little glimpses and reminders of what things around me really look like, outside of me emotional connections. And what depersonalization felt like was that feeling written across the world: I was in a strange room, a strange house, a strange town, a strange world. (Incidentally, I was in a strange town, in an unstable housing situation, but that doesn't quite explain how I felt.) It was like I was walked into a house that was decorated similar to my house, but was not exactly my house. I intellectually knew that it was, that everything was the same, but the weight of associations was suddenly and inexplicably lifted.
The whole thing is easy to explain in these terms, but woefully inadequate. Looking at what I wrote now, it seems silly that it changed me as much as it did, that a week of feeling something as theoretical as "my emotional associations with my surroundings are no longer there" could become the worst week of my life. (Although I should point out that dental pain added another element of misery to this).
One thing should be said about this: certain aspects of this don't sound too terrible, and at times they are not. This is not an angsty or existentialist experience of asking myself how did I get into this shotgun shack?. This is also not a sudden breath of newness that makes us question the structures we have built up for ourselves. It bears passing reference to those things, in perhaps the same way that Tabasco sauce bears a resemblance to actually burning your mouth with a red hot piece of metal. This has an immediacy and unpleasantness to it that makes it hard to compare with other things. In other words, although it might bear a slight resemblance to experiences that are not all bad, derealization and depersonalization are not the new rock and roll.
My experience went away for two reasons. First, much of my situational problems were solved. I found support, got healthier, and had a root canal, and slept a whole bunch. Second, I believe that (at least for me), there was a nadir of confusion that was hit and there was a natural recovery from it. I don't know, and my experience is not objectively or subjectively standard compared to other's experiences. I don't know if it a self-resolving and limited problem for everyone. And right now, as much as I have written this down and objectively remember that I felt subjectively terrible, it is hard for me to recapture just why I was so afraid and confused when the feelings of derealization came across me.