I kept a journal for almost 1 year and then mostly stopped. Not because it was not useful but because it had done what I needed it to do and continuing felt like going through the motions.
I should say upfront that I am not a natural journaler. The first few months were genuinely painful. I'd sit down with the intention of writing something meaningful and produce two sentences about being tired and not knowing what to eat for dinner. Which, rereading it now, is actually kind of charming. But at the time I thought I was doing it wrong.
There is not a wrong way to do it. That's the first thing I learned.
The second thing I learned is that you cannot trust your own memory. I was certain, absolutely certain, that a difficult period at work had lasted most of a year. I went back through the journal and it had lasted 8 weeks. Not trivial, but not a year either. My memory had inflated it into something almost mythic because of how hard it felt. The journal had the receipts.
The third thing is that I had a lot more good days than I remembered. Good in small ways, a conversation that made me laugh, a piece of work I was proud of, a meal that was just right. But good. And I'd completely forgotten most of them.
That one changed something for me. Not in a gratitude-journal, count-your-blessings way. Just a factual recalibration. Things were pretty okay, actually. They had been for a while.