Explain why "the colossal significance of that light had now vanished forever" and why gatsby's "count of enchanted objects had diminished by one."

Because he had lost Daisy he realizes that the importance and significance of his dream has died a little bit. She chooses Tom, even after all he has done for her, so her place as an enchanted object in his life has dulled and disappeared. It's a sad realization, but justified by what has happened.