My wife will think of my ex-girlfriend every once in a while, and then she will be unhappy. What should I do?

This shows that your wife has no confidence in you, or you have not given your wife enough sense of security. In this case, I think it is better to communicate frankly. I can comfort my wife that my ex and I are a thing of the past, and it is completely impossible in the future. Then tell her that he is your wife and you should live a good life in the future. People who have lived your whole life need not worry about anything. After a good communication with your wife, I think it should be much better.