invasion of privacy (reading someone's journal)

I know it sounds extremely childish but I keep a journal I don't have many friends and I used as a way of venting and relieving anxiety for making decisions on my own. It helps
However my S/O took it upon himself to read it one day and read some things about my past and my thoughts about him after first meeting him. He brings it up all the time (it's nothing I did wrong just a simple feeling I USED TO HAVE).
So I stopped writing in it for months. I felt ashamed to even read what I had written and what he possibly could've read my twisted sad brain on display I felt stripped of my identity (sound dramatic but true) so I decided to get a new journal one he couldn't recognize. I left for the store one day and he had found it in a hiding place and read it all.
It's such an invasion of privacy and it makes me feel really awful. How do I make him stop with out haveing to stop journaling? 
Or should I just give up the journaling because its childish?