I think that some things should be private, even in a marriage, and even when there is nothing to 'hide'. I think it is unfair that she is going through your journals, but at the same time I think it's important to tell her how you feel about it, quietly, in a way she can understand, like explaining to her that this is your coping mechanism and you don't want to have to censor yourself. If she feels like she can't resist snooping, I would recommend you lock the journals away if possible.
This is just my response from how I would feel if I or my husband were in this situation, but every relationship is different - maybe other people will have other answers for you. This just happens to be my particular viewpoint.