Some of the scariest modern research, at least for me, is the stuff that looks into how the brain works and how it affects the body. The person I thought I was, that creation with Free Will, is largely imaginary. The real me is a mix of chemicals and neural pathways that have little to do with choice and a lot to do with repetition. In real life we don't choose very often, at least not with anything like free will. That makes resolutions sort of superficial, dependent on whether our brains will go along. Usually they don't.
For one thing, the brain doesn't understand past and future. It operates in the present, which is actually pretty lucky. Can you imagine what your life would be like if your brain could actually recreate the emotion you felt at the worst point in your past? We'd all be suicidal. To protect us, our brain puts that away, so that while we remember how sad it was or how scary or how painful, we can't experience it again. Likewise, the brain doesn't let us anticipate future pain or sorrow fully. We know it will be bad, but it's in a "mirror darkly." That quality can be a problem when we try to change our lives. If the brain doesn't know the future, how can it understand that it's not going to get those M&Ms it loves so much any more? It doesn't. It tells us it wants what it had before, and most of the time, eventually, we give in.
So how do we change, really change? It's a battle between your brain and your will, and honestly, your will is at a disadvantage. It's fighting physical symptoms (sent out by your brain), habits of long standing, and neural pathways that are short and well-traveled. It takes a great deal of awareness of the present (where the brain is) to counter these factors. The reason resolutions fade is because we lose that awareness (I'm not a smoker anymore) as day-to-day activities intervene and the brain is allowed back on auto-pilot. Sometimes we're even conscious of the slippage and promise "I'll exercise twice as long tomorrow," but of course your brain doesn't "get" tomorrow.
So what works? Constant reminders, continual monitoring, and lots of support from others who know what you're trying to accomplish. Although the brain is actually amazingly adaptible, research shows that it loves to do what it already knows how to do. If that includes overeating, vegging on the couch, or other rotten habits, you need to nag it into accepting another way of life. And isn't that what a new year is for?