Final answer:
The woman is experiencing the generativity versus stagnation challenge in middle adulthood.
Step-by-step explanation:
The woman in her mid-40s who feels that she has failed to do anything substantial with her life and decides to go back to college to earn a nursing degree is experiencing a generativity versus stagnation challenge, which is a task of middle adulthood.
Generativity involves finding one's life's work and contributing to the development of others, such as through volunteering or mentoring. By going back to college and pursuing a nursing degree, the woman is taking steps towards generativity by seeking to contribute to others through healthcare.