My boyfriend went through my messages

So yesterday my boyfriend went through my snap messages. I got mad at him over it but he was upset with what he found. He found messages of me flirting with other guys before we were dating (before i even knew my boyfriend) on obviously this isn’t a nice thing to see so he is upset. I understand why he’s upset but he also shouldn’t have gone through my messages and scrolled that far back. What should I do? I’ve said sorry and im so upset that he feels sad. I don’t know what I should do.