When a saved person commits sin, that sin is already under the blood in reference to eternal life. Colossians 2:13 states, "And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses." All trespasses (or sins) have already been forgiven us. You see, before God saved you, He knew every sin you had ever committed but He also knew every sin you would commit. He gave you eternal life with that knowledge and forgave you of all your sins at that moment.
This goes back to the method by which we are saved. God does not save us by giving us a clean slate that we now must keep cleaned up in order to make it to heaven. God saves us by making a switch. He puts our sin on Christ and He puts the righteousness of Christ on us. 2 Corinthians 5:21 says, "For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." Christ was made sin for us so that we might be made righteous in Him. [By the way, what if God only put your past sins on Christ? He would have to put each new sin on Him as you confessed it. Certainly, this did not happen. Christ paid the entire debt for sin on the cross. Each person who believes in Him enters into forgiveness through Him.]
You may recall that the New Testament often speaks of the believer as being in Christ or in Him. You see, when the Father looks on us concerning our eternal condition, He does not see us as standing in our own righteousness, but in the righteousness of Christ. Concerning Christ, Paul said that he wanted to be "found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith" (Philippians 3:9). When we trust in Jesus Christ by faith, we are placed into Christ and are seen by the Father as having His righteousness.
Now, you may ask, why do we need to ask for forgiveness of sins at all if they are all forgiven? The reason is that the present forgiveness of sins which we seek on a continuing basis deals with something entirely different from salvation. Though we are saved eternally by our position in Christ, we have a walking relationship with God in this life that is based on obedience and submission to His will. Even though we can never cease being saved, we can cease to be in a good present relationship with God.
Let me give you an example. When my children were little, they would sometimes disobey me and get in a wrong relationship with their father. I might even be angry with them and use some form of correction with them. However, there was nothing they could do, no matter how bad, that would make them cease to be my children. They were flesh of my flesh. To deny them would be to deny myself. Their family relationship to me was based on their birth (just as our eternal relationship to God is based on the new birth). Yet, they (just as us with God) could be my children and still not have all things right with me. They never ceased to be my children and I never had to make them my children a second time, but there were many times that we had to work out a right relationship that had gone sour. This is the way it is with God.