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 What allows to make a choice and What is volition?
Easy. Well, easy in theory :) -- you need to get fed up! You need to get really sick and tired of your conditions, so sick that you cannot live like that anymore. You are so fed up that you are either ready to die - or to change your habits. So that desire to get out leads to iron intent to change.