There's an interesting study of the military veterans from the Vietnam War who came back as heroin addicts (a lot of them). They had about an 85% success rate in getting clean long-term. Vs. a much, much lower rate for most addicts.

One of the primary reasons is suspected to be just the complete change in lifestyle when getting back to the States. None of the usual triggers for using exist anymore. Usual dealers are gone. The stresses of war are gone. The buddies to shoot up with are gone. Etc. While most addicts usually eventually return to the same patterns that trigger use.

The lesson is that the a key to beating a bad habit isn't necessarily brute force willpower. It's avoiding the need to even use willpower by just making it easy for yourself.