In a series of four experiments, Loran Nordgren from Northwestern University showed that people suffer from a "restraint bias", where they overestimate their ability to control their own impulses. Those who fall prey to this fallacy most strongly are more likely to dive into tempting situations. Smokers, for example, who are trying to quit, are more likely to put themselves in situations if they think they're invulnerable to temptation. As a result, they're more likely to relapse.
Thus, not only do people overestimate their abilities to carry out non-immediate plans (far-mode thinking, like in planning fallacy), but also the more confident ones turn out to be least able. This might have something to do with how public commitment may be counterproductive: once you've effectively signaled your intentions, the pressure to actually implement them fades away. Once you believe yourself to have asserted self-image of a person with good self-control, maintaining the actual self-control loses priority.