How does Myrtle's character change from the garage in the Valley of Ashes to the Manhattan apartment? And how does this connect to the American Dream?

Myrtle changes from a disgruntled wife of a mechanic into a high flying party girl in New York City. Myrtle knows she is merely Tom's mistress but she buys into the illusion of the American Dream, at least while she is on her alcohol fuelled tryst with Tom. Her dream, of course, comes crashing down when she is forced back into reality.