Nonduality is irreversible because it’s the place of total reality. It’s the place of who you are, who you’ve always been. It’s so obvious that you can never unsee it once you see it, once you be it. Because you are awake. You’re realizing that you are the person seeing it. You are who is being it. You’re the master of your life; you’re the person driving your vehicle of life.
The illusion of separation is very, very, very thin. You have to work constantly at keeping it on yourself. So, the second you stop doing that, you’re there, and that’s why it’s irreversible.