Funny, I had the same thing with my ex up until our last year together when suddenly she was very secretive and didn't want me to EVER go into her emails on our computer. Very strange since I didn't care to snoop. But, her insistence made me curious... found nothing of interest, but still. Not a good way to live. And, since I am knowledgeable in Computer Forensics, even the deleted stuff was easy for me to see. (yes, even deleted photos on the camera were mind numbingly easy to see)
I don't snoop on my GF phone or computer and she doesn't snoop on mine. If she has something she does not want me to see, I don't care to find out. I don't want to go back to my old relationship, I want complete trust. To get complete trust, one must offer complete trust. Besides, once you marry, there are few if any secrets. And if you really don't want your SO to know something about you... perhaps you should not get married.