Since I was a teenager I have kept a notebook. These I have filled sporadically not so much with internal reflections, but quotes from books, fiction and non-fiction, poetry, the occasional sketch, and minutiae like to-do lists, questions I want to explore, facts I'd like to remember, etc. Some years very little has gone into a notebook. Other years I have filled several.
Having had several bouts of clinical depression in the last decade, I decided about a year ago to start a separate diary. The intention there was to do more self-reflection (and not of the interminable looping self-critical talk that tends to pervade my stream of consciousness, especially when depressed!) and to perhaps put into words thoughts and feelings that I have trouble getting a firm grip on in my head alone. I also use this to record general day to day activity if I want to. I complete each entry with a list of 3 to 6 things I'm grateful for on that day. I added to this a daily practice of 10 habits I attempt, not always successfully, to engage in each day, and tick them off. I've found this combination of reflection, locating things to be grateful of, and maintaining intentional habits, to really assist with combating my depressive tendencies. (And to give direction and combat inertia at times).
The start of my journaling also coincided with my early suspicions of ASD and eventual diagnosis. It in many ways assisted with the process of diagnosis too, and I'm sure, my processing of it.