Hello… It’s easy to forget details such as a person’s name from decades ago, especially when you probably didn’t use her name frequently across a wide range of situations — her name didn’t come up often in a lot of conversations. I don’t know of any techniques to help you recall her name that are practical (sometimes returning to a place can help you remember such details).
It might help to tell your wife that you can’t remember her name because you didn’t talk about her to others or think of her very often. Apologize to your wife that you can’t remember such details and stress that you’re willing to do whatever it takes to make her feel loved and valued.
Hope that helps.