I noticed that a lot of days/weeks/months started to blur together as I got older (and I’m not that old).
I started keeping a very factual journal each day.
Every hour or so, I write down where I was and what I did—subjective impressions are optional.
It’s been really helpful in jogging my memory and seeing where my mind was at a few months prior.
I just do it in a note program on my phone, so I can fill it in while walking, in the bathroom, etc.