Margot slit the thin, white string with her utility knife and sighed. Any resolve she had mustered earlier that morning had long since faded. As she retracted the slender blade into its metal sheath, the quick onset of mental calorie counting started to carve a deep crevice between her well-sculpted brows.        The pink lid popped open, and the aroma of freshly baked pastries filled the air around her.      Her spine straightened.      For Margot, the excuse for falling off the sugar wagon always came right after feeling guilty.      She had been working her tail off for days. The idea of even one more spinach-and-hemp-seed smoothie made…