When I was twelve or thirteen, one of my older sisters started collecting the Diaries of Anais Nin, and I started reading one (eventually read them all). I was fascinated and started keeping a journal of my own. Through my teens, I wrote fairly regularly, although there were stretches of weeks or even months, when I wrote nothing.
At some point in my early twenties, I suppose when I was moving from one place to another and boxing up journals, I was leafing through them and was stunned to find not only descriptions of things that I had completely forgotten, but descriptions of things that I thought I remembered, but for which my memory was very clearly false. It was tremendously unsettling because up to that point I'd assumed, as most people do, that my memory was generally reliable.
By then, my journal writing had settled into a pretty steady pattern. For decades now, there are not more than a dozen days in any given year that I don't write something (although in the years when I was courting Lynn, most of that writing went into letters to her). And I keep a rough daily log of where I've been, who I've had meetings with at work, what restaurants I've eaten in. When Lynn asks, as we're getting ready to return to a city we visited years ago, "And what was that restaurant we ate in our first night?" I can find out. And I'm always amused, and sometimes astonished, to see the ways in which my memory has diverged from the evidence of the written record.
Elizabeth Loftus has done the most accessible work on the fallibility of memory. Her research shows repeatedly how unreliable memory is, and how much of what we think we remember is internally constructed. The fact is, we have no internal way of distinguishing between true and false memories. They feel exactly the same. Eventually one learns to assume that all of what we think we remember is inaccurate in some degree.
Most people believe that their memories get worse as they get older. That's not as true as they think it is. Over time we just accumulate increasing evidence of how poor and unreliable our memories have always been.
Unfortunately, most people don't know this. The most negative impact, of course, is the reliance on eyewitness testimony in criminal trials, even though it has been shown again and again that for any given event, the testimony of eyewitnesses will invariably diverge, often very significantly. But on a very personal level it affects our very sense of who we are, if we believe that our sense of self is an accumulation of our memories.