Privacy in a marriage

My husband has this journal his therapist wants him to write in. It's to work out trauma (which he's never told me about). She said if he ever wants her to read anything to just give it to her on the page he wants her to read. Now to me, I have went through my partners phone. If I feel something sketchy I feel I have the right to look. I feel like it's the same with a journal. He can have another women read it but not his wife? I think I should have a right to read it whenever I want. I tried being nice and asked to read it once and he flipped on me.