In the play "A Streetcar Named Desire" Blanche ends up being institutionalized in a mental ward. Many things lead to her final breakdown.
Perhaps the most prominent thing which led to her breakdown was Stanley's rape of Blanche. The sexual tension between both Stanley and Blanche built throughout the entire play. Blanche would display herself in front of him as a taunt.
Another thing which lead to Blanche's breakdown was the fact that Mitch decides to leave her. Mitch admits that there were certain things about Blanche which he knew she had shaded (a reference to the lamps and coverings she placed over them to disguise her age), he could not forgive her for lying to him about her prostituting past.
The final nail in Blanche's figurative coffin was the fact that Stella had left her once again. Blanche never forgave Stella for leaving their childhood home. Blanche even blames Stella for losing Belle Reve- though she had nothing to do with it. Blanche destroyed Belle Reve in the same way which her family had- trying to use it to support them financially.
In the end, Stella decides to choose Stanley over Blanche. This decision propels Blanche over the edge.