Post PRK: eyesight keeps regressing drastically after 8 months of correcting high myopia, what may be the cause?

Regression after PRK can happen but looking at the amount of laser that was done on your eye one can only presume that your cornea has thinned out quite a bit.  You might have developed a condition we refer to as post laser ectasia.  That has to be rules out first and as quicly as possible before you attempt any further correction.

It is not usual for such drastic regression 8 months post PRK.  You should see a Lasik/PRK surgeon to reevaluate your cornea topography.  It sometimes can be caused by post surgical corneal ectasia.