Newsflash: You don't.
What exactly did the blood of Christ do and how are sins dealt with? Also, what role does the High Priest serve? These are questions that once you find the answer to you come to realize the total forgiveness of sin that you have through the shed blood of Christ, and His role as our High Priest/Mediator.
You'll find out that the forgiveness of sin (or rather, remission of sin) happens through bloodshed (His sacrifice) and Jesus as our High Priest intercedes on our behalf forever, being a mediator between man and God. He is the propitiation for our sins, the appeasement. You'll also find out that Jesus doesn't do daily sacrifices to deal with daily sin like the priests of old, but having an eternal priesthood did one sacrifice for sin for all time.
Consider how the sins of new converts are dealt with. Through the blood of Christ, the shedding of His blood. His eternally sufficient sacrifice for sin. Now think, if sin is dealt with through bloodshed, how is your sin dealt with? Your answer is found in Christ, and His role as our High Priest.
Read the book of Hebrews to find all of this out, and more.