How can we avoid forgetting our spouse's kindness after many years?

By remembering that rights run both ways, not one way.
In the Farewell Pilgrimage sermon, the Prophet ﷺ declared: you have rights over your wives, and your wives have rights over you — and he mentioned among their rights that they be treated well in clothing and food. He also enjoined good treatment of women. A simple practice: name one thing you are grateful for in your spouse every day, specifically. See Rights of husband and wife.