When a person makes the decision to stay clean and sober (at least for the day), it can be a major problem to do so. After all, they are generally surrounded by friends who are into the altered state life style. They often have tons of their drug of choice lying around, and not wanting to “waste” such a promising supply can be a major passive barrier to qutting for real. And on top of that, there are underlying desires (such as escape and fun) which just make it that much better to use than it is not to use. When it comes right down to it, sticking to sobriety is about as easy as the life style you rebuild.
Rebuilding the way that you live your life is about as easy as changing your weight, either through proactively building muscle or through burning off body fat. In either case, it is never easy. There are just too many easy ways to relapse. In most instances, you have both a life which has thus far been built both to be drunk or high as often as is humanly possible, and a life in which every person you interact with on a regular basis delights in you being as far off the wagon as they are (which is usually pretty far).
Of course, it is far from impossible to stick to sobriety. You basically just have to get rid of your relationships with all of the people who would like you to be drunk or high, and get rid of (without using) all of your remaining supply. If anybody actually cares about you (and you might be surprised), have them do the second part when you are not around. Just tell them where all of your stashes are, and then let it happen. It will suck for awhile, but you will eventually get used to it.