Constantly remind yourself that they can't remember.
The seemingly simple task of remembering they can't remember will cause a paradigm shift. Do not read anything into it. They really just can't remember. Take the time to consider what that means. It means they cannot follow your lengthy explanation of what short-term memory loss is. It means they have lost their ability to manipulate you. They cannot be trained or taught to take their medicine or drink their water. They cannot remember what you said no matter how loudly or slowly you said it. Their forgetting does not diminish the previous value of any person, relationship, experience or thing. We do not want to believe that we are so easily forgotten. We may get resentful and angry. We have an intense need to be acknowledged and remembered. Usually we view it as their anger lashing out with a combative spirit, when, at least in part, it may be ours. They simply can't remember. How much of the Alzheimer's/dementia struggle is about us? How much of their struggle comes from our anxiety? We need to be willing to move toward the unknown.
Accepting this is accepting their loss and our loss. It represents a shift in the relationship that we may not be ready for. That acceptance may be heartbreaking, but it is the starting point for care.