weak-willed (spec. relating to the inability to avoid acting against one’s better judgment) adj., n. akratic

[Having this condition is also known as acrasia.] An akratic is a person who knows the right thing to do but can’t help doing the opposite. This is not just [British Prime Minster] Boris Johnson to a tee. It is also the reason why he embodies more than anyone else a Brexit project in which the very people who promised to take back control are utterly incapable of exercising it, even over themselves. (Fintan O’Toole, “The Ham of Fate,” New York Review of Books, 08/15/2009.)

This word can also be a synonym for undisciplined irresolute, feeble-minded and undisciplined.