A sign that it's time to step back and rethink what you're doing:

if you catch yourself trying to pare leather using your own thumb as a cutting surface.  (Fortunately, I stopped myself in time.)

The problem at hand: for the book I'm binding for the upcoming New England Guild of Bookworkers exhibition, I have a leather octopus whose head and body are decorating the front cover as cushion onlay, most of the tentacles are serving as sewing supports, and the tips of those tentacles are decorating the back cover as cushion onlay.  Unfortunately, I didn't finish the edge paring before I sewed the book.  Finishing that paring now that the leather is attached to the book is way harder than I thought (though why I didn't think it would be this hard, the world may never know).  In particular, those little tentacle tips are really difficult to 1. reach at the right angle while 2. keeping them on a firm surface and 3. holding them still.  Adding in constraint 4. no peeling myself (whose importance I'd momentarily forgotten, it seems), I've wound up with holding the tentacle tips against  a small glass vial, and thinning the edges with a bit of sandpaper wrapped around my index finger.