Folks, whenever you see a maze in any book, you can bet that what you're getting is a great big ol' SYMBOL from the author, in all-caps. What better symbol for being confused or lost could there be?
It's no different here.
Lou's teenage experience in the maze surrounding Stortford Castle is deeply traumatic and shapes her life in a profound way. Now, the book never gets too explicit about what happened that night: Lou was partying with her friends at the center of the maze when suddenly all of the girls disappeared, leaving her with a group of drunk and disorderly men. This is where things get hazy, but it's heavily implied that Lou was raped. This explains why she describes it as the "exact day [she] stopped being fearless" (12.1).
From this day forward, Lou is never the same. She stops being outgoing. She loses her desire to leave her small town and explore the world. She stops doing anything that might be construed as provocative. This is because she still blames herself for what happened, partially because she's never been open about the experience with others.
That is, until she re-enters the maze with Will and has a full-on mental breakdown. Will then comforts her until she feels comfortable enough to reveal her story. More than that, however, he tells her something she really needs to hear. Take a look:
"You don't need me to tell you that it wasn't your fault," he said quietly.
"Yes. Well. I still feel...responsible. I drank too much to show off. I was a terrible flirt. I was–"
"No. They were responsible."
Nobody had ever said those words aloud to me. (17.249)
This is an important thing for Lou to hear. While that terrible night was indeed traumatic, she's let it hold even more power over her by blaming herself for what happened. By finally sharing her pain with someone—someone who doesn't judge her and instead supports her unconditionally—Lou has taken the first step on the road to healing.