the tiny amount of dextrose in the artificial sweetener is not the thing keeping you from losing twenty pounds. first of all, the last twenty lbs will come off the slowest - know that. but it's going to be about the amount of calories. even if you think calories mean nothing, and only carbs matter (oy vey), the dextrose is still providing an absolutely trivial amount of carbs.
track your calories. the primal blueprint, mark's book, makes no bones about it - you need to be in a caloric deficit to lose weight.