<human>: We have been together for nearly 3 years and have a place together.

We were both gutting the house and I found a letter in amongst OUR stuff. I skimmed my eyes over it to check it's importance before chucking it. A few words caught my eye and I read most of the letter. It was a letter from an ex dating 2005. His ex had wrote that she could never forgive him for cheating (he swears up and down he has never cheated) and that she is sad she miscarried. 

I was really shocked by this and immediately wish I never read it. He walked into the room and the first thing I said was "You cheated on Fiona and got her pregnant!?" He wasn't angry at first and explained that he didn't cheat, he left her and started seeing someone else. And also said that she was never pregnant it was all a lie. Then he got mad.

He said I invaded his privacy and I'm clearly paranoid and want to snoop on him. He was also angry that I 'grilled' him about cheating on her. He is completely ignoring me now.

I apologized profusely and I am genuinely sorry that I upset him. A part of me feels like this is an over reaction as he has left ex pictures and intimate letters in our flat as if its no big deal, but because I read one I have ruined everything. For the record, he doesn't hang on to these as keep sakes. He just has a few old pictures and letters in boxes full of other things from his past. 

Am I definitely in the wrong for doing this?

Give me a shortened version of the text
<bot>: I read a letter from an ex dated 2005 that was amongst OUR stuff and asked him questions about it. He is now very angry and said I invaded his privacy. Don't know what to do.