What part/quote of The Great Gatsby best highlights the fact that Gatsby is living in the past/obsessed with recreating his past and being with Daisy again?
I think that Chapter 7 provides some of the best examples of Gatsby's love of the past with Daisy inserted into it. Some of the best evidence of this can be seen in Gatsby's exchange with Tom. When Gatsby declares to Tom that Daisy does not love him and that her real love is with Gatsby, one sees the desperation with which Gatsby clings to both the past and a conception of Daisy that might not exist: "She never loved you, do you hear?” he cried. “She only married you because I was poor and she was tired of waiting for me. It was a terrible mistake, but in her heart she never loved any one except me..." Gatsby speaks with a declaration, but it is also tinged with a love of the past, and what that past might have meant in his own mind, another vision of the "Platonic conception of self." The only difference this time is that no one else comes to believe it.
As the conversation progresses, it becomes evident that Daisy cannot commit to him. Gatsby recognizes that his own vision of the past is not what Daisy shares. It is at this moment that he recognizes the frailty of his own obsession, the flip side "of a promise that the rock of the world was founded securely on a fairy's wing.”
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, undespairingly, toward that lost voice across the room.
For Gatsby, this is a moment in which the collision between the past that lives in his mind and the present that is staring at him collides. The result is that he strives "to touch what was no longer tangible."
This unhappy struggle between past and present can be seen at the end of the chapter. Given what had happened, Gatsby goes back to a role of protector. He seeks to serve as lookout for Daisy, a silent watchman or protector over her. As Gatsby is standing watch, “in the moonlight—watching over nothing," it becomes clear that he still reveres her as he did in the past. Gatsby speaks of how they have a signal worked out if Tom "tries any brutality." Gatsby's love for Daisy and the past that she represents is still there, even as she is working out plans with Tom for her getaway. The collision of images speaks volumes: "The sacredness of the vigil" and Tom and Daisy "conspiring together." The close to the chapter represents how much Gatsby lives in the past and how he wishes to recreate his past with Daisy in it.