"I do not know what the future will bring, but it cannot be as beautiful or as satisfying as the past." (11.16)
Ashley sums up the book's attitude neatly: The past was always better. It's worth pointing out that this pretty much assumes that you're a pro-slavery Southern white person.
Oh, there were so many things she would preface with "Do you remember!" …And while they talked she could perhaps read in his eyes some quickening of emotion, some hint that behind the barrier of husbandly affection for Melanie, he still cared […]. (15.10)
The memory of the world before the war is here linked directly to Scarlett's love of Ashley. It's almost like the way she hangs onto that ideal past is by pretending she loves the doofus.
When she arose at last and saw again the black ruins of Twelve Oaks, her head was raised high and something that was youth and beauty and potential tenderness had gone out of her face forever. What was past was past. Those who were dead were dead. The lazy luxury of the old days was gone, never to return. And as Scarlett settled the heavy basket across her arm, she had settled her own mind and her own life.
There was no going back and she was going forward. (25.49-50)
Scarlett's strength comes from not looking back… or does it? She says she's not looking at the past, but her determination to go forward comes from looking at the ruins of Twelve Oaks. Running from the past doesn't exactly mean you're not paying attention to it or thinking about it.
"Ah," said Melanie sadly, "what will the South be like without all our fine boys? What would the South have been if they had lived? We could use their courage and their energy and their brains. […]"
"There will never again be men like them," said Carreen softly. "No one can take their places." (29.70-71)
Carreen is specifically remembering her fiancé Brent Tarleton here. She's also getting at the tragedy for the South in having an entire generation of young men destroyed. And she's linking that to the lost past—the deaths in the Civil War end up consecrating the pre-Civil War South.
"It's a curse—this not wanting to look on naked realities. Until the war, life was never more real to me than a shadow show on a curtain. And I preferred it so. I do not like the outlines of things to be too sharp. I like them gently blurred, a little hazy."
"In other words, Scarlett, I am a coward." (31.90-92)
Ashley explains that his fascination with the past makes him a coward. Is this true? Or is it his whining on about how he's a coward that makes him a coward?
The old days had gone but these people would go their ways as if the old days still existed, charming, leisurely, determined not to rush and scramble for pennies as the Yankees did, determined to part with none of their old ways.
Scarlett knew that she, too, was greatly changed. […]
She could not ignore life. She had to live it […]. (35.177-179)
Here, the suggestion is that the old days are not exactly gone; instead they live on in those who remember them, and who maintain their morality. Moving forward, as Scarlett does, is a betrayal because the Yankees are the future. The past is a faith, which Scarlett wishes she shared but doesn't.
"They don't talk of anything else," thought Scarlett. "Nothing but the war. Always the war. And they'll never talk of anything but the war. No, not until they die." (41.168)
Usually the past in the novel is the world before the war, but here it's the war itself, which everyone talks about endlessly. Scarlett's not interested in either, though. She always said the war bored her.
"It isn't losing their money, my pet. I tell you it's losing their world—the world they were raised in. They're like fish out of water or cats with wings. They were raised to be certain persons, to do certain things, to occupy certain niches. And these persons and things and niches disappeared forever when General Lee arrived at Appomattox." (43.109)
Rhett argues that the white Southerners they know aren't sad about losing their money, but are sad about losing their place. This is supposed to make them more noble. But is it true? The places that have been lost, the niches that Ashley and everyone were to fill—they were all made possible by money, and property, and specifically by slave labor. The Southern way of life they're all mourning is impossible without slavery since it was created by robbing black people of their liberty and their labor by force.
"Yes, life has a glitter now—of a sort. That's what's wrong with it. The old days had no glitter but they had a charm, a beauty, a slow-paced glamour." (53.73)
Hey, Ashley—it's easy to move at a slow pace when all the work is done by slave labor.
"He loved the child, Scarlett, and I guess he drinks to forget about her." (60.15)
Rhett is mostly presented as not being in thrall to the past as many of the other characters are… but his collapse after Bonnie's death is an exception. As with Ashley, his inability to move beyond his memory destroys him. Scarlett can keep going, but the men in her life consistently falter.