Yes, absolutely I believed it. It made me extremely apathetic, because what was the point of doing anything if it was just going to be a waste of time? I found the idea that the world could have any kind of concrete existence just too awful to bear. I saw suffering everywhere and it was comforting to believe that it was not "really" happening, surely the world couldn't be that cruel?
But yes, I did believe it, and often still do when I'm in the pit. I can have diametrically opposite beliefs at the opposite poles, and each time I dismiss the memories of the other pole as a "what was I thinking?"
It can get hard to keep on top of the delusions and it often takes quite a bit of my attention. I've gotten used to continuously checking myself, sometimes three or four times to make absolutely sure. I would sometimes make arithmetic mistakes in something with a lot of digits, because I would see the wrong number the first time I looked. I basically have to turn a critical eye on absolutely everything all the time. It does get very exhausting, but I've been doing it for so long that it's an automatic habit.
Before I started taking lamotrigine, the intensity of the moods would be such that they eventually overrode my discipline and that's when things started spiraling and other people noticed strange behaviour. Now they're mild enough that I haven't had a psychotic episode since April.
It seems so bizarre when I write it down. It feels like brain incontinence