You can't read Paul's letters for very long without discovering that God intends to change our natures. Paul says anyone in Christ is a new creation, (2 Cor 5:17), and that we died to sin in Christ (Romans 6:2-6), and that we have been crucified with him (Gal 2:20).
These verses have always made me think that for a Christian, overcoming a bad habit or a recurring attitude should be fairly effortless. All we need to do is have faith and ask God to change us, right? In fact, the change should already have taken place.
But I've never found change in my life to be effortless, like these verses imply. I've figured out in recent years that Scripture has another paradigm besides the total and instant transformation. The story of God's people coming into the Promised Land is a surprisingly complicated one. There are miracles (crossing the Red Sea, conquering Jericho) that feed the instantaneous, effortless paradigm. But there are also a lot of enemies, that have to be conquered one by one, and in not all cases is the victory supernatural and instantaneous. The biggest delay, the 40 years in the wilderness, comes because the people lose faith at a critical moment, but I'm not sure the story should be read that if the people had only had faith, all would have been effortless. What if God intended the process to be somewhat drawn out, because he wanted to give the people the experience of having faith in the promise, even when circumstances suggested that the promise couldn't be true? Faith is the assurance of things not seen, Hebrews tells us. That means something has to remain unseen for faith to take place.