crucified the flesh with its passions and desires.”
Scripture says that those who belong to Christ Jesus have crucified (past tense) the flesh with its passions and desires. No one can commit suicide by crucifixion. Someone else must nail us to the Cross. The flesh cannot crucify itself. Jesus has already done what we could not do. The sooner we stop trying to crucify ourselves, the sooner we can boast in the finished work of the Cross and actually witness the evidence of it in our lives.
Notice that Paul does not merely say that Jesus was crucified for us. That is certainly true in and of itself. But there is more. Not only was Jesus crucified FOR us, He was crucified AS us. God considers us to have been crucified as surely as if we were there receiving the nails in our hands and feet. We have been crucified together with Him because God placed us in Him the moment He was crucified.