"I am crucified with Christ: nevertheless, I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me and gave himself for me."
“I am crucified with Christ:”
“I am crucified — literally, ‘I have been crucified with Christ.’  This more particularizes the foregoing.”
Paul understood that when the Savior died on the cross, the believer’s “old man” died with Him.
“Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin” (Romans 6:6).
“That I may know him, and the power of his resurrection, and the fellowship of his sufferings, being made conformable unto his death” (Philippians 3:10).
Therefore, believer… “I am dead.” 
I am dead to the Law.
“For I through the law am dead to the law, that I might live unto God” (Galatians 2:19).
I am dead to sin.
“What shall we say then? Shall we continue in sin, that grace may abound? God forbid. How shall we, that are dead to sin, live any longer therein? (Romans 6:1-2).
“Nevertheless I live;”
Paul said that he is alive and active for Christ, not a dead man that did or felt nothing! He has a beautiful relationship with Christ, and he responds to Him through His Word.
“Yet not I, but Christ liveth in me:”
Paul knows that his life is not about himself but about Christ, who lives in him.
“And the life which I now live in the flesh I live by the faith of the Son of God.” This life is a life of faith with a targeted focus—Jesus Christ! I trust Him!
“So then they that are in the flesh cannot please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his” (Romans 8:8-9).
“Who loved me, and gave himself for me.”
“Jesus loves me, this I know, for the Bible tells me so.” In the most excellent demonstration of love that could ever be, Jesus is the One who gave His life as a sacrifice for me, to pay for my sin-debt. Surely, He is the vine, and we believers are the branches receiving our sustenance from Him. He is in us, and we are in Him (John 15:1-8).
We believers alive from the dead can serve our Lord through His grace, and in obedience to His Word.
 Jamison, Fausset, and Brown. Jamison-Fausset-Brown’s Commentary, the electronic version in eSword.
 JFB, ibid.