If you fill your body with junk food and empty calories, you'll gain weight and feel lousy. Eating healthy foods, getting plenty of exercise, and drinking lots of water will boost your energy and help your body burn calories more efficiently.
Whether shopping or thinking about food; approaching the table with a plan or with trepidation; or talking about what you want to eat or will try not to eat, dieting can be sort of exhausting, just from the sheer concentration of it all. Yet -- ironically -- food is your best tool to gain and maintain energy, resolve, and clear thinking.
You just have to do it right.