"[Daisy's] eyes fell on Jordan and me with a sort of appeal, as though she realized at last what she was doing – and as though she had never, all along, intended doing anything at all. But it was done now. It was too late." (pg 126) - Nick
Gatsby spent his entire life building his dream. Gatsby's completion of his dreams and his success all depends on having Daisy.
His ability to get Daisy depends on her unhappiness in her marriage, her undying love for Gatsby, and her willingness to exchange her entire life for a life of "love."
The above quote is thought by Nick when Daisy hesitates to reply that she "never loved [Tom]"(126) , and shows that she has been lying to Gatsby about never loving Tom.
Gatsby's dream world that he created for himself comes crashing down when he echoes Daisy's words "You loved me too?"(126)
This quote also shows that while Daisy told Gatsby that she never loved anyone else, and that she would leave Tom for him, this was never her intention.
"But with every word she was drawing further and further into herself so he gave that up and only the dead dream fought on as the afternoon slipped away, trying to touch what was no longer tangible, struggling unhappily, toward that lost voice across the room." (128)
Gatsby becomes more and more aware that Daisy would never be his. The dream that he once had of them being together is slipping away.
Daisy and Tom
This is where Tom breaks Daisy down and when all the confidence that Daisy had of being with Gatsby leaves her.
She does love Tom and did love Tom, no matter how much she’d like to deny it. And even tells Gatsby that "even alone [she] can’t say [she] never loved Tom," because "it wouldn’t be true." (126)
As shown through the above quote, despite her flirtations, Daisy never intended to actually leave Tom.
"Daisy and Tom were sitting opposite each other at the kitchen table[...]They weren't happy[...]yet they weren't unhappy either. There was an unmistakable air of natural intimacy about the picture." (138)
Daisy has almost everything that any woman could want out of a marriage - she is very wealthy, she has a beautiful daughter, and although there are affairs on both sides, she has a comfortable marriage with Tom.
At that time, divorce was also very uncommon, and it was very unlikely that any woman would leave her husband for any reason at all.
Despite all that she said to Gatsby of her love for him, she would never trade all that she had in her marriage to Tom Buchanan for Jay Gatsby.