Why do people backslide with new years resolutions? With smokers literally seeing them go up in smoke? The reason is because smoking and over eating and other unwanted behaviors are deep rooted habits. Willpower alone has an uphill battle when it comes up against habitual behavior which is in all cases fueled by emotion and imagination seated in the subconscious mind. You must have cooperation of the whole mind and not just your conscious decision to break a habit because in that act you are literally fighting with yourself to control your behavior.
So you fail and feel guilty or doubt your ability to succeed one more time. With subconscious cooperation you can empower your conscious mind to assist you in behavior change rather than try to force those changes to take place.