A Christian can defeat himself in two ways: one is to forget the holiness of God and the fact that sin is sin. The Bible calls us to an ever deeper commitment in giving ourselves to Christ for him to produce his fruit through us.
The other is to allow himself to be worn out by Christians who turn Christianity into a romanticism. The realism of the Bible is that God does not excuse sin, but neither is he finished with us when he finds sin in us. And for this we should be thankful.