Final answer:
In middle age, when one mourns lost opportunities and wonders about the road not taken, they are experiencing the psychological phenomenon of integrity versus despair.
Step-by-step explanation:
When middle-aged individuals mourn lost opportunities and wonder about the road not taken, they are likely experiencing a psychological phenomenon known as integrity versus despair. According to Erikson's theory of psychosocial development, late adulthood is a stage where people reflect on their lives and either feel a sense of satisfaction or a sense of failure. Those who feel regret and focus on what could have been may face feelings of bitterness and depression.