Here's how to make the cheat work:
In the mirror room position yourself so that you're halfway
inside the right mirror (the one leading to flower garden).
Then tap with the "+" key 3*short, 3*long, 3*short. Your lifebar should change to infinity. If this doesn't work then just tap + randomly until it does.
When the code is on you can turn it off by pressing + in the mirror room.
If you press it somewhere else than mirror room, you skip to the next screen.
You can also enter the "out of order" mirrors now.