Many failures of self-regulation occur when we fail to pay attention and allow ourself to operate on autopilot if it is a habit, or allow our Inner Horse to run wild if it is a low probability emotional behavior.
Like physical strength and endurance, willpower is a limited resource. You are more likely to engage in behaviors which you later consider manifestaitons of a weak will when your willpower is fatigued. this is one reaosn you are more likely to screw up when you are physically tired, stressed out or already doing something requiring willpower.
Failures of self-regulation also occur when there are not clear cut standards. You will not carry out a plan if you lack a clear goal.
Self-regulation also seemingly fails when goals, values, motives change - when what we call self changes. This constantly occurs in politics. The goal of the Republican President may be to cut spending on environmental protections and infrastructure. Then a Democrat comes in, reverse the policy, only to have policy reversed again as a Republican takes office. It makes intuitive sense that both nations and individuals with more consistent leadership are more propserous.