Blanche is a woman in a state of inner and outer decay. She is the shadow of her former self, and the antithesis of all she had hoped to become.
She is a woman who once enjoyed the most desirable wants in life: beauty, status, money, company, freedom. She was a gorgeous lady, married to a husband she loved, and member of a prestigious Southern family.
With time, all the very things she was made of began to fall apart: Her husband turned out to be gay, and killed himself. The sound of the shotgun still resonates in Blanche's mind and leaves her paralyzed with astonishment each time she remembers it. This shows a post-traumatic stress disordered mind which has lost more than just a husband.
As the family fortune turned around, Blanche lost now 2 symbols of status, money and marriage. The lack of both happened too suddenly for her to be prepared for such a fall. Hence, he hangs on to the only thing she has left: looks.
Since her reputedly good looks are all that she has of remembrance from the woman she was once, she hangs on to them with such exaggeration that sex became her power tool. Yet, she abused it. She slept with one of her students, and her reputation tarnished for good.
Running on empty, she fled to her sister's dysfunctional family home pretending to still be who she used to be: Haughty, proud, snobby, and still lusty and lecherous. Yet, there she is finally finished off by Kowalski, who openly shows the world who she really is, rapes her, and then institutionalizes her.
In the end, she was a trainwreck with no destination. That was her sad truth.