Not my personal account (no IFR rating), but
I found out the difference between simulated IMC and the real thing during instrument training for my CPL(H), which is of course more extended than that for the private pilot's licence. I was wearing foggles, and we had actually erected screens as well. I was quite certain that I could not see anything outside, and I was focusing completely on the instrument panel -- or so I truly believed. After a short period I began to get the sort of spatial disorientation discussed above that is so common in instrument flying; I felt as if I was in a steady right-hand turn, although my instruments told me that I was flying straight and level. It was annoying, but basically not a problem and I just carried on flying.
Then suddenly my perception of turning increased dramatically. I felt as though I was in a very steep turn, going almost round in circles. I began to get dizzy and nauseous, and I found it hard to concentrate. I told my instructor, and I said that if it continued I thought I'd have to stop. But then, just as suddenly, the feelings of turning lessened, and I assured him I was alright. My instructor was more put out than I had expected. '(Name)', he asked me sharply, 'Are you quite sure you can't see anything outside?'
I assured him that I was absolutely certain. I had only been able to see the instruments for the whole of the session ... or so I thought. He then told me that the short period during which I had felt so ill had been the exact time during which we had flown through a small cloud. I had had no idea of this, but it seemed that my body and senses had somehow perceived the difference between simulated and real IMC.
Pilots who have never flown in real cloud are often unaware of the dramatic difference between this and instrument training. Flying in cloud can be totally, completely disorienting. (...)
Krasnar, H. Flying helicopters. A companion to the PPL(H), p. 134f.