How is called the word that means to have an intention or wish so weak that he/she decides not to do something. What I remember is that this word it seems have common roots with the word volition and it is formal.
A number of words come to mind. irresolute if the intention is never fully established because the person is hesitant or uncertain of what course to take, or vacillating if the person keeps going back and forth between options. There is also, of course, weak-willed.