I recall being similarly despondent at the same 8mg point when some morning pain and fatigue surfaced. @johnbishop 's recommendation to record a pain level each morning (1-10 scale) was one of my guiding factors, as was my own observation that there was sometimes some slight achy-ness for a week or so after reductions, which passed.

Everyone is slightly different and will make their own assessment, but I increased dose for a few days to 9.5mg, then dropped to 9mg before resuming the tapering.

PMR takes a while to resolve and prednisone is not curative, it just holds inflammation and pain at bay while the body recovers from the PMR. Maybe you had reduced a little too quickly, maybe it will take a week or two to adjust to the new dose, or maybe you need a small increase to keep the inflammation/pain controlled.