Honestly, this is a common anxiety. A lockbox or mini-safe of some kind is a possibility, too, although there's a certain loss of convenience that comes with that. Digital journaling is also an option from a security standpoint, but definitely loses something as compared to physically writing, at least for a lot of people.
I'm a physical pen-and-paper girl myself, so I get the concern. When I was younger (like 16), on one occasion someone decided to have themselves a peak, and gave it away by the timing of something they said to me. From then on, I started carrying it with me in my backpack at all times.
Nowadays, I live with people who won’t disrespect me in that way, and that makes it a lot easier. So, as I like to say in response to this relatively common question: protect yourself while you have to, then surround yourself with good people once you’re able.
There’s also various locking journals pretty easily available on amazon or elsewhere, one of which I actually have. (This one actually: https://www.amazon.com/Password-Notebook-Stationery-Business-Multicolor/dp/B071CW17R7)
Now, I have no illusions whatsoever that this would actually keep anyone out that had even a little determination, but for anyone civilized, it should suffice just fine. Really though, I got it more because I think it looks cute… being able to lock it is at most just ritualistic if anything. (For that matter, my wife actually knows what my code is anyway, but I also completely trust she won’t read it, so it’s fine.)