Stop and clean EVERYTHING!!! If you really have scratches, you're getting them with larger grit from somewhere. Scratches will still show up after polishing the mirror out. Of course, you could be getting the scratches from storing the glass or from bad grit. If you have a plaster tool, make sure that there isn't any grit under or around anything. If necessary paint the tool, or at least the holes, and grind off the paint from the tiles as necessary with the next wet of grit. When placing the mirror on a new wet, take a few strokes with the mirror just barely touching the tool by lifting the mirror a bit to break down the larger grit clumps that may be in the grit. Also, if you put the grit on dry, wipe the grit around with a clean finger before putting the mirror down. Before starting polishing, the whole mirror should have a nice frosty finish. You might like to use a 5-10 power eyeloupe to look at the surface to make sure that you really have scratches or just marks on the surface of the mirror. Fingernails make scary looking scratches on the surface but are really just some nail material. Remember that you are getting to the point where you have to be clean or you mess up.