The problem with diets and such nonsense is that just like when you try to quit smoking, it's not just the mental addiction to nicotine/food that's the problem- it's that the body, even the brain reconfigures to "deal" with the poisons you put in your body when you smoke/indulge in bad foods/poisons.
The body is always trying to reach a state of homeostasis (balance), and when you quit anything you're addicted to like food or smoking, there are going to be some systems that need "realigning"- especially your mind (not so much brain as mind/mental state).
When your body reaches it's best defense posture (so to speak) to the poisons (the best it can do given it can't stop you from indulging), this "new" state is going to line up with your addiction to some degree and you will feel somewhat balanced but when you stop doing exactly what your body wishes you'd do- it goes into shock. This is because it's not "configured" physically nor mentally (your mind) to instantly adjust to the "better" state.
This doesn't mean the second you stop your body isn't frantically cleaning house and readjusting things- that's what the withdrawals are all about- it's the body's state when it's reconfiguring.
Think of a store. Let's say your grocery store moved ALL of the products around making your built up speed and efficiency impossible. Your "easy" shopping trip becomes frustrating. Not impossible, and in time you will get accustomed to where everything is again in the new configuration. If you never get over being angry over them moving everything around, however, what's healed?
Healing the body is a great first step but healing the mind, the very way you think about foods and your health is the vital one that makes even a healthy body stay that way.
Try shopping through chaos like the above example and you'll get the beginnings of what is happening when you "diet" and why the pain is so great it often makes people go back to their old habits.
Thing is, sadly, if you stick it through, this may not even work as well unless you have anthony robbins next to you giving you advice the whole way- thing is, your mind is still in the wrong set, it's thinking about food backwards and has associated pleasure to poisons (like sugar!).
Again, you haven't dealt with the real problem. Your body's addiction (something there is no quick fix to renovate other than giving the body the tools- but giving someone a hammer is no guarantee they can/will build an entire house right?).