Why is it so hard to stick to a diet?

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Ive been trying to lose weight my whole life. I try to watch what i eat and walk around the neighborhood for at least thrice a week. But there are days that I just wanna take a break and stuff my face with all my favorite foods - chocolates, french fries, etc. Sometimes I give in to these cravings and then feel bad aftwards. I wanna be disciplined with myself so I can succeed with losing all the excess pounds. But i dont know how. What do you guys usually do when you feel like giving in to your cravings?