Critiquing is the way to learn to edit.
This is what I’m learning right now.
I’m doing a critique right now of an epic fantasy (with the associated epic length). Every time I critique a novel, I learn something new, and today several things that were rolling around inside my head came together.
A novel has a heart, a core truth that the rest of the story hangs on, that every part of the novel relates to, focuses the energy of the story towards. You might call it the theme, but it might not be related to the theme. The heart of Lord of the Rings, for example, isn’t really related to the Good Triumphs Evil theme, rather to the truth that you can’t go home again, that change changes everything, including you. Neither Frodo nor Bilbo nor the Elves nor the Orcs nor even Aragorn could return to the lives they once led, as much as they wished to.
Your job as an editor/critiquer (should you choose to accept it) is to find the heart of the story in front of you. Like JRR Tolkien showed us examples of how each character in his trilogy couldn’t go home again, couldn’t take up like what happened hadn’t changed him or her, everything in your story should circle around its heart, like planets in a solar system circle the sun.