Brian Laundrie's Notebook Revealed, Admits Killing Gabby Petito

Brian Laundrie's notebook confession to murdering Gabby Petito is now public for the first time ... revealing his handwritten excuse for why he took her life.

In the 8-page confession, first obtained by Fox News, Laundrie admits killing her, but claims he only did it after Gabby sustained an injury while they were hiking in Wyoming.

He thought what he was doing was "merciful, that it is what she wanted, but I see now all the mistakes I made. I panicked. I was in shock."

Laundrie also seemed to be ready to take his own life at the point of writing his confession ... saying "From the moment I decided, took away her pain, I knew I couldn't go on without her."

As you know, the notebook was picked up with the rest of his bag in Florida's Myakkahatchee Creek Environmental Park by FBI agents back in October when the hunt for him was in full swing.

The pages suffered some significant damage, although his writings remained legible -- it had been underwater for roughly 5 weeks.