I keep a diary myself — recording what happened and how I felt each day, because memory fades and blurs, especially the feelings of the moment.
The app I use is Logseq, an open-source note-taking tool with Markdown support. No sign-up needed; everything is stored locally, keeping your notes private and under your control (cloud notes are a hassle to export).
My favorite part: the file you open each time is named with today's date, so you just write whatever — even a plain log, which is exactly what I do. That removes any pressure to categorize; you can end each entry with a # tag and entries auto-group for easy search.
To sync between computers, use Nutstore (坚果云). For phone–computer sync, the best I've found is Syncthing. If Logseq's built-in search feels weak, try AnytxtSearcher. Logseq also supports plugins — my two favorites are Heatmap (a contribution heatmap; blank = a day you skipped, easy to go back and fill) and RandomNote (revisits a random note).