Hold on, let me get out the essay I wrote on it last year...
Ha, found it..
Well, the Valley of Ashes where Myrtle lives is supposed to represent fallen hopes and destroyed dreams.
yeah, the green light represents hope.
Daisy and Myrtle are actually types of flowers. A daisy it al tall, regal flower. It comes up before most and it white - purity. Myrtle, however, is a low, ground covering plant with these tiny flowers, either periwinkle or purple flowers. The idea is that myrtle tries to be flashy and beautiful, but it can never match the high-class daisy.
I wish I could help more - I aced this part of my American Literature class, but I just cant remember everything about the book. If there are certain items you need help with, I can help. I just need to refresh my memory of what's actually in the book.