Found this out when I was 18. I had some tightly-held flashbulb memories that I had built my entire self concept around. All of them were either so mutated as to have no reference to reality, or were provably completely fabricated. Suffice it to say I did not take this realization well. Did not help that I learned this three days after I had proven to myself that my entire belief system was 100% running on the placebo effect. I could have dealt with each of these fine if I had been able to deal with them individually, but getting hit with both of them like a hook-uppercut combo just destroyed me for six months. I suppose this is to be expected from having your identity scientifically disproven; I count myself lucky to have recovered with only a crippling fear of advertisements that I was able to work through in a mere four years.

Just to be clear, I'm fine now. This is all a distant memory, but it is to me where my current life began.