I suppose that mid-life could be seen as the opening of the issues of the last act of life. Certainly Jung seems to; remember this?
In the secret hour of life's midday the parabola is reversed, death is born. The second half of life does not signify ascent, unfolding, increase, exuberance, but death, since the end is its goal. The negation of life's fulfillment is synonymous with the refusal to accept its ending. Both mean not wanting to live, and not wanting to live is identical with not wanting to die. Waxing and Waning make one curve.
Midlife can be a time of stress as emotion breaks through ego boundaries – reflecting that which feel injured or neglected.
“Emotions are not chosen; they choose us and have a logic of their own.” James Hollis
One person may experience the fear of losing control and the sense of self that once worked. Another may feel the fear of further losing areas of self-expression. Frequently, there is the existential fear of mortality and diminishing time, the realization that half of life is gone.
It is common to experience anger or depression in response to lost time and opportunity for more authentic experience. Depression and underlying regret may reflect an emerging sense of emptiness and the superficial relationship to life of the “adapted self.”
These are calls to attend to life issues which have been neglected. As Jung said,
We cannot live the afternoon of life according to the programme of life's morning, for what was great in the morning will be little at evening, and what in the morning was true will at evening have become a lie.
In drama the first act is used to establish the dramatic situation and introduce the main characters. At the end of the first act, an inciting incident complicates the story and moves the screenplay into the second act. -- This is childhood through young adulthood, when we set the stage for our lives, choose our work and relationships.
The second act, commonly described as "rising action", typically depicts the protagonist attempting to solve the problems caused by the inciting incident. The Climax, which ends the second act, is the scene or sequence in which the main tension and dramatic questions of the story are brought to their most intense point. -- This is the time from 35 or so to 55 or 60, what has classically been known as midlife.
Finally, the third act features the resolution of the story and its subplots. It is the third act that I am becoming most interested in, the time in which life's loose ends, unresolved plotlines, the denouement of life.