Within the canon of the story, that journal is the one Thistle's been keeping since she first left home. A while ago, I lost every single one of my sketchbooks that I had filled from age 14 to 26, and that was a pretty heavy blow. I imagine this feels the same.
For the curious, panels 6 and 7 are flashbacks to this page and this page, specifically. Originally, I had planned to write in all the notes shown in the way the book had first appeared, but Elizabeth Tadehara on Patreon pointed out that it was much more profound blank - and I agree!