Myrtle Wilson is ashamed of her social position in life. She is ashamed that she is married to a poor man who Nick describes as "a blond, spiritless man, anaemic, and faintly handsome." Myrtle tries to compensate for this shame by acting proud. She treats her husband, George, terribly in order to assert herself as being better than him, like a wealthy aristocrat rudely ordering around a servant.
At the apartment, Myrtle is away from that life she is ashamed of. She is with Tom, a more successful man, and feels freer in this situation. She acts with hauteur in this case because she's indulging herself in this, for her, alternative world of higher social status.
Her laughter, her gestures, her assertions became more violently affected moment by moment, and as she expanded the room grew smaller around her, until she seemed to be revolving on a noisy, creaking pivot through the smoky air.
Myrtle changes her clothes when she is at the apartment. This is part of her social change from poor George Wilson's wife to a socialite having cocktails with Tom Buchanan and friends. Myrtle was playing up this role while she could. She was indulging in this alternative to her life with George. It is hard to sympathize with Myrtle because she is unfaithful to George and because she acts so superficially, but she feels trapped in her weary life. This world with Tom is an escape from her drab world in the Valley of Ashes.