Since the very first New Year's resolution, and subsequent realization that resolutions rarely work, people have understood that habits are hard to break. Once you're used to doing something, it's hard to change. So what's the deal? Why can't I just decide to stop eating Snickers bars everyday after coming home from work? Why can't I get over that nougetty goodness? Well, it all starts with habit.
Habits are any action that has been ingrained in your mind, to allow your brain to perform it's daily functions more efficiently. You see, the brain has a lot of things to process, and so anytime it can save energy or concentration, it will do so. If your brain starts to notice a pattern in your behavior, it will start to formulate the response in the form of a habit. Once your brain does this, it's very hard to break it, as long as you're being exposed to the same spurring event or cue. Some guys smarter than me have proposed that cues stem from the fulfillment of any of the various parts found in the Hierarchy of Needs proposed by Maslow.
Whether it be physiological, safety, loving, esteem or self-actualization, most habits can be linked to one of these needs, as your mind identifies the cue (the need) and the response (the way you fulfill that need) and habitualizes it.
Now, that is also the solution to breaking a habit. You must first identify the need, keep the cue the same, but change the response in a way that the need is still being met. As long as the reward for fulfilling the need is deemed worthwhile, your brain will trade out the new habit with the old one, if the cue remains the same. Suggestions by Dr. Phillippa Lally and her associates claim based on recent research that automaticity of new habits formed after about 66 days of repeated and conscious action on average. So, if you feel like you haven't noticed a change, keep going. Breaking that habit and making a new one, can sometimes take time. Don't give up, and realize that even if you take a day off, it won't affect habit formation if you start back up again. Just be consistent, and be the change you want to see.