Answer:
Nancy's lapse in memory is due to substance abuse
Step-by-step explanation:
While Amnesia will result from damage to a brain structure in the limbic system, responsible for emotions and memories, often known as neurological amnesia.
Nancy has rather a self-caused forgetting which is enabled by
use of drugs like benzodiazepines or sedatives, alcohol, marijuana often causes forgetting effects yet amnesia is a disorder that generally speaking arises as a result of a brain injury or a severe mental health conditions
Today we know that certain substance called beta-blockers help destabilize painful or fearful memories that ofter are hard to forget.
Persons that are suffering in a relationship can have certain periods where they solve substance abuse and taking alcohol with drugs often leads to a substance called metyrapone that blocks the synthesis of the cortisol.
The stress hormone cortisol is suppressed allowing the person to "forget those painful moments".
Since many people solve to drugs and their negative effects are severe, by no means, this will be a solution.
It would be always advised to seek help and gain better insight to deal with circumstances cleverly.