Another way of describing this or explaining this is by asking who is ruling our life. Before a person becomes a Christian he or she is full or ruled by himself or herself.When a person is saved or become a Christian or become born again, Jesus enters or is admitted in his or her life and we call this the new man or new nature. Our old self do not like the new self so the old self bullies our new nature.
Basically this struggle between the old and new nature defines our Christian life. I believe this is a daily battle.
To win most of our battle we have to depend on Jesus and fill our lives with God's words and pray that the Holy Spirit prevails in our lives. One of the things i observed is when we have spiritual victories through the Spirit the old nature tries to get credit and if we fall for it, we then experience defeat and that starts a cycle.
Our old nature works through the flesh or physical while the new nature works on the spiritual or strengthened by the Spirit.
The more of Jesus we have the more our victories. Let this sink in in us for this time.