How we cite our quotes: (Act.Scene.Line)
Quote #1
DELPHI: It's tough to live with people stuck in the past, isn't it? (1.6.37)
Delphi is the villain of the play, both because she wants to resurrect Voldemort and because she kills an innocent boy. But like the play's heroes, she too wants to change the past so she can see her father again.
Quote #2
HARRY: You're fourteen years old now, Albus, and I wanted to give you something which—meant something. This…is the last thing I had from my mum. The only thing. I was given to the Dursleys wrapped in it. (1.7.20)
Harry is still thinking about the past, and he's passing the weight of it onto his son, who never asked for it. The "people stuck in the past" describes Harry Potter with a capital P.
Quote #3
HARRY: I was dreaming—I was under the stairs and then I—I heard him—Voldemort—so clearly. (2.2.9)
Even Harry's subconscious is stuck in the past. His dreams are almost identical to his memories, with a little Voldemort added in for good measure. It's almost like even his subconscious knows that if he doesn't stop obsessing over his past, bad things will happen.