Top 2 reasons changing a food behavior
can feel impossible to do.

Reason 1:  Addiction

Reason 2:  Biology

You have millions of cells and neurons and trillions of bacteria on the inside and outside of your body - each talking to one another to determine what you need to sustain yourself in the current environment.  Generally speaking, these cells and bacteria don't want your environment to change because they believe the environment is required for survival.  So they ask you to keep doing the same behavior over and over and over and over again so they can survive.  Yep, really.  Your cells don't know consuming too much sugar is a problem, they simply receive what you have given them, adjust, store excesses, and move on.  Your body is designed to consume large amounts of carbohydrates because it was often a very limited resource.  So when the body knew when carbohydrates were available, it needed to take in as much as possible.  And it did.  The body doesn't have an off switch.  If it did, we wouldn't be able to survive.    So when you try and make a change, they remind you to go back and keep doing what you were doing....don't change or we might die.
There is actually research that shows us how we can become and remain addicted to food products.  In books such as Salt, Sugar, Fat by Michael Moss and Fast Food Nation by Eric Schlosser, we can see how food manufacturers have actually developed something called "The Bliss Point," which makes food chemically more palatable for continued consumption - aka addictive!  Neurologically and physiologically our bodies love the bliss point and use every excuse to get us to engage in behaviors to get the bliss point.  Thus, the inherent problem with any form of processed food is becoming addicted to ingredients, which the body no longer recognizes as food.  Instead it is a drug of choice the body wants to keep receiving.  You will have to repeatedly increase the quantity of the food item in order for your body to feel like it has reached an adequate amount to satisfy the craving.  For example, you might eat one peanut butter cup and be satisfied, then it will take two to feel equally as satisfied, then three and so on.  Your body will want to fight any changes you decide to make to this behavior.