I've kept diaries before. I tore up the one when I was a teenager out of fear someone would find it and read it. That one was so angry and vicious toward myself.
The one in my early twenties was mostly about loneliness and me being so sad and feeling awkward & guilty for having an intense crush on a guy. I forced myself to reread that one a couple years ago to see if I was repeating the same mistakes. When I wrote my emotions felt so serious & dramatic, I felt in despair. Rereading it, it just sounded so shallow and juvenile, I was kinda put out with myself (I had hoped I was more interesting than that). I ended up shredding that one too.
One thing I worried about was if something happened to me and my family members would get my possessions. The idea of them reading that horrified me. I wrote about issues with my parents. I can't imagine how bad my mom would feel reading that. Lots of ranting in it. Hardly anybody in there was written in a positive light because I didn't need to write about that- it's the negative stuff I wrote about, to get that stuff out.
I've also experimented with learning and writing backwards so no one could read it. But then I had trouble rereading that so I thought what's the point.
Now I just write down nuggets from whatever I come across- lines from books or movies- that make sense to me or comfort me. I have several I've found here on psychcentral.