I've kept a daily diary a few times for a short while, but usually as a way to monitor myself when I'm taking medication or having counselling etc., so really only impressions of my mood, without much detail of events. If I tried to do it every single day as a matter of routine, most days would just say; "same again".
I have always kept a lot of scribblings and jottings though; they're a pretty random collection, but they do span my life - letters from friends, notebooks with my draft replies, school magazine articles, drafts of song lyrics, flyers from gigs I went to, notes to take to therapy sessions etc. I have found many of them fascinating to read again since my autism diagnosis; the growing awareness that I was different to other people is very apparent, and I was more accurate in my insights into the working of my own mind than I ever gave myself credit for at the time. Some of the more personal song lyrics from my early twenties perfectly sum up "help, I'm an overwhelmed autistic person having a burn-out!".
One of my therapy prep' notes in particular made my jaw drop; it was from my first course of CBT, when I was in my mid 20s; over two decades before an autism assessment was suggested. At that time, I knew almost nothing about autism, nor had any inclination to find out, and would have laughed at anyone who suggested that I was autistic, despite how badly I was struggling. Yet, you could almost tick off one-by-one all of the autism diagnostic criteria, and a whole load of other autistic traits and experiences that I only learned about since joining WP and other online communities. It describes most of my social problems, poor executive functioning, sensory sensitivities, and my inability to judge how any of these compared to the people around me.
I does make me wish that I'd made an effort to write more down. I'm certainly glad that I have hung on to the stuff that I have, even if some of it does make me cringe with embarrassment!
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When you are fighting an invisible monster, first throw a bucket of paint over it.