"The Blessing Tobacco," what does David try over and over to do on the way home from his grandmother's house, and what is the significance of this?

After smoking many cigarettes, David feels nauseous, and on his way home he tries several times to vomit, but he cannot make anything come up. David feels as if "so much" is trapped deep inside his gut and he just wants to rid himself of it (110). His sickness is more than literal--David's circumstances in life have created trauma that feels like a physical entity trapped inside him, and he just wishes he could free himself of it.