The turning point of the novel comes in Chapter VII, when Tom and Gatsby confront one another. The confrontation begins when Tom antagonizes Gatsby and bates Gatsby with questions and insinuations about his past.
For a while, Gatsby is cool, collected and in control. It seems that he will win the day and win Daisy. When Gatsby declares that Daisy never loved Tom and demands that she corroborate this claim, things begin to turn against Gatsby. His demand is too much for Daisy to sustain.
“Oh, you want too much!” she cried to Gatsby. “I love you now—isn't that enough? I can't help what's past.” She began to sob helplessly. “I did love him once—but I loved you too.”
At this point, Gatsby's hope to erase the last five years are dashed. Daisy simply cannot go through with that kind of effacement. Her life with Tom has meant something to her. She has loved him. To say otherwise is to negate the meaning of her life, to some degree, and she cannot do this. Even if her reasons are different, the result is the same.
Daisy cannot agree to erase the last five years and Gatsby's hope to repeat the past becomes, rather quickly, a very real impossibility despite his confidence and his sense of destiny. Reality, here, begins to set in. Gatsby will not win the day. He has asked for too much.
Tom continues his attack on Gatsby and takes the upper hand after Daisy relents in her preference for Gatsby alone. He brings up Gatsby's bootlegging and other details of his illicit business dealings. Daisy is surprised and disturbed.
Not long after this episode, Myrtle is run down in the "death car" by an upset Daisy Buchanan and the novel speeds toward its close.
This chapter offers a number of quotes that may be taken as representative of the turning point of the novel, but Daisy's hard-pressed confession of loving both men seems a proper summation of the dynamic that leads to the novel's decision.
It's not enough for Gatsby to have Daisy love him. He needs an absolute rejection of her past with Tom in order to be satisfied.