Daisy says that she loves Tom when it matters most to her future with Gastby.
At the very crux of her crisis, Daisy is put upon to declare who she loves. Gatsby demands that she say that she never loved Tom.
“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.”
Gastby's plan is to begin anew. To meet this end, he feels that the past must be wiped clean, obliterated; made to have never been. This is, of course, impossible. The impossibility is not merely scientific, however. Daisy cannot bring herself to declare that her marriage to Tom was meaningless. She cannot claim that there was no honesty in it, ever, and that it was a sham or a false show.
For Daisy, the marriage has to have had some integrity, even if she was unhappy and Tom was unfaithful. Denying this, she will be undoing a significant part of her life.
Failing this denial is unacceptable to Gatsby. He needs to hear Daisy say that she never loved Tom. When she says that she loves them both, she effectively begins the destruction of Gastby's dream.
After this point, Tom seizes the advantage, the party breaks up, Daisy runs over Myrtle and the novel's main action comes to an end.