The main thing is to not judge yourself. Journals and diaries are personal little extensions of your mind to hide away anything you want. It's your personal mini therapist and message board. Keeping journals is a beautiful and creative thing. It's not a writing assignment so what you write doesn't have to be serious or even make sense. it doesn't even have to be organized.
I think maybe you need to spice it up a little by adding more writing to it, get more comfortable with writing down your thoughts about everyday things, hobbies and events.
Try to get creative with it as well. What I do is add cool quotes on the tops of the page. I even doodle to visually represent something I saw or dreamed about.

Idk I'm a complete journal junkie so I'm not trying to sound so crazy I just love journaling

Edit- also if you are afraid of someone reading it, what about writing it in a different language or writing it backwards? Do you have any good hiding places? It's a legitimate worry but it's not something you are helpless to prevent.