Christianity is not the faith of the complacent the comfortable or of the timid. It demands and creates heroic souls like Wesley Wilberforce Bonhoeffer John Paul the Second and Billy Graham. Each showed in their own way the relentless and powerful influence of the message of Jesus Christ.
We may think there is willpower involved but more likely... change is due to want power. Wanting the new addiction more than the old one. Wanting the new me in preference to the person I am now.