What is the hardest thing about dieting?

Adherence . Adhering go a diet and sustaining it is probably the most difficult. So when choosing a diet, find it as a lifestyle change and complement it with sustainable behaviors and exercise regimen so it does not DIE soon!!
Deprivation. When you tell your body that it can't have certain foods, it wants them even more. Instead be more reasonable about eating a variety of foods in moderation. By making positive, small changes gradually, you will avoid this. Examples: adding more water, adding more veggies & fruits, switching to whole grains, putting the fork down between bites, avoiding distracted or secret eating. Baby steps!