Saying "I've changed" is a meaningless thing. How did he change? Did he take time to understand why he did what he did? What did he learn about himself? Did he tell you what character flaw led him down that path? What life changes did he make to ensure that he would never be tempted to do that again?

If the only thing he can point to are wholly external things, cutting contact with his affair partner, giving you access to all his electronic, those aren't really him changing anything. He didn't tell you about the cheating. She did. She either felt guilt for doing it or she was retaliating against him for breaking it off. Either way he didn't feel guilty for having done what he did, only for getting caught.

If all he can give you are the words "I've changed", you're insecurity is valid.

Now what? I think your boyfriend and you should consider couples counseling and your boyfriend should discuss why he did what he did.