I dont see how you managed to get that much of a dip. Did you run out of compound? Or maybe you didn't put it down deep enougth? Or have you lost compound through a gap in the subfloor? You can sand your high spot but the compound dries rock hard. You could also loose the dip in your glue, but im guessing if you managed to get a dip as bad as that you may find loosing a dip that deep in tile adhesive even harder to do.