What Christians have all understood is how Jesus was crucified for us and how His death was for the punishment of our sins. But Paul then speaks about two other crucifixions. He says, "Knowing this, that our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin" (Ro.6:6), and "I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself up for me" (Ga.2:20).
Our old self refers to our old sinful nature because of which we used to sin. Whenever some temptation came to us that enticed us to sin, this nature agreed with it. So, we would generally give in to sin. But when we came to Jesus we repented (turned away) from our sin and decided we didn't want to sin any more. That was when this old self was crucified. God gave us instead a new self with His nature in it that hated to sin because it was the nature of God (1Jn.3:9).
Paul then takes it to the next level. Now it is not merely about not sinning, but about whose will we do, ours or Christ's. Now we can place ourselves on the cross as crucified with Christ and declare that it is no longer our will we want to do, but Christ's. This is "Not my will but Yours, Lord," in continual action. In another place, Paul uses another image to depict the same meaning, talking of placing our bodies as living sacrifices in order to be able to continually do God's will in everything (Ro.12:1,2).
Now we can understand that if we have not entered in our life into all the three crucifixions, our Christian life is never going to be what it ought to be. We may wonder what is wrong, or blame different things for our lack of fruit in our life, but this is the way for us to have a proper relationship with God.
Sadly, a large number of people have only heard about Christ dying for them. They interpret this as if everything was and will be done by Christ, and they can simply enjoy the benefit. But having said that they are still far from being happy. Naturally. If we don't have a hatred for sin in order to stop sinning, and we know that there is forgiveness for us if we sin, won't we take sin carelessly and continue sinning? If we want to stop sinning but we don't see the challenge of our will being crucified in order to make way for God's will to be done in our life, we will not be able to enter the kind of life Jesus came to give us.
There are things God has done for us. He has paid for our sins so that we need not face judgment, and He has regenerated us and given us a new heart that does not want to sin. But there are things we need to do in response from our side. We must take up our cross daily and deny ourselves in order to do God's will (Lk.9:23). We must also have a determination in our heart and mind that it is God's will alone that we must do in everything.