How is it possible to change habits cultivated over lifetimes

If one has cultivated a habit for countless lifetimes, how is it possible to reverse such habit? Is it possible at all? Is that person entirely at the mercy of external factors? (are we ever not entirely at the mercy of external factors? This is an entirely different question though). I have trouble understanding how habits are changed, and where that change originates. I figure the answer to this 'extreme case' question will shed some light. Thank you. EDIT: This question is a different way of phrasing these two questions https://buddhism.stackexchange.com/questions/18716/what-allows-to-make-a-choice and https://buddhism.stackexchange.com/questions/18738/what-is-volition/18762#18762