How does it change from when she is at the garage to in New York?

I can give you a general coment. 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.