I've kept a diary fairly regularly since 2001 - though there have been gaps, notably large parts of 2020. I used to use an A6 day-to-a-page volume, but that got a bit daunting. For 2019 onwards, I bought a five-year diary, and I do like looking back at what I did manage to write. It's been especially useful in tracking J's development and and my concerns which have eventually led to a diagnosis.