You are not alone. Most humans are just that human. We make our promises/decisions, but when the going gets better; we break them.
There can be many reason but it comes down to the fact that we as humans, forget. Yes, we forget on purpose as it might become difficult to keep our commitment because we are not as strong in our self discipline as we should be. Self control is hard for some. Let me use something here: Let's say a person has a weakness. They are not a bad person but like most of us, we have a weakness. Sometimes these weaknesses can bring us a lot of pain and destruction. We get in mess that needs to be fixed. We cry woe is me and somehow we get the help we needed from God, another person or life things and we carry on and we can breathe again. Down the road a block or two; we are tempted with this same weakness or vice (whichever) and we do not have the strength to stay away from what has caused us the pain and horror. Our Self discipline, Self Control or being able to just say no to a friend or foe or whatever the weakness is; has just been shot out the window. Here we go again and if we do it enough we can wind up loosing a lot, which might never be regained without a lot of work and sacrifices.
When we make decision to change our life for the better; it is seemingly natural to fall back every now and then but one has to have the right lesson; to not fall off the bandwagon. Once you experience the right lesson; you will be wiser and it will be easier to stay on the decision to do whatever it was...or not do it again. We all need more self control and self discipline, having someone to help you with these qualities might be the answer. Meaning a Life Coach. Sometimes there may be reason that you might not be able to see yourself; as you are asking this question. Another persion might be able to help you pinpoint why you do this. Personal growth is a part of life whether we do it with self help or from someone else.
At least you can say what you are missing; that is a first step. Most people do not recognize or care...they just keep on trucking and then they hit a big pothole.