What we know of Daisy's state of mind right before her wedding is something we learn from Jordan Baker, who relates her story to the narrator, Nick, in Chapter IV. We must remember that while Nick seems to be a fairly reliable narrator, Jordan Baker is shown to us as an "incurably dishonest" (63) woman according to Nick. It is difficult to know how truthful her account of the scene before the wedding really is.
According to Jordan, Daisy gets "as drunk as a monkey" (81) the night before her wedding to Tom Buchanan, shortly before the bridal dinner. This is the first time in her life she has ever had a drink, so it seems likely she could really could be that inebriated. She is holding onto a letter in one hand and a bottle of sauterne in the other. Tom has given her a very expensive strand of pearls, and she has thrown them into the wastebasket at some earlier point. Now she retrieves them from the wastebasket and flings them away, telling Jordan to give them to someone downstairs, whoever would like them. Jordan and Daisy's mother's maid manage to get Daisy into a cold bath, to sober her up, but throughout most of the bath, Daisy persists in hanging onto this letter. Finally, it begins to fall apart, so she is persuaded to leave it behind. At no point are we told what is in this letter. They manage to get Daisy dressed and get her to the bridal dinner. If Jordan Baker's narrative is true, we can reasonably infer that this was a love letter from Gatsby to Daisy and that Daisy is regretting her decision to marry Tom. Nevertheless, Daisy gets up the next day as though none of this has ever happened and marries Tom, "without so much as a shiver" (81). When she returns from the honeymoon, Jordan reports to Nick, "I thought I'd never seen a girl so mad about her husband" (81).
Is Daisy still in love with Gatsby? Is it Tom's money and his being a more conventional person of her own class that makes her make this choice? This scene strongly suggests that both are true, and that Daisy has sold out, as she does later in the novel, to her own version of the American dream, so unlike that of Gatsby.