Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
Note 4 at Ro 6:12: If this sentence were to be diagrammed the way we were taught in school for the purpose of identifying the subject and verb, then the understood subject of this sentence would be "you." Paul was saying, "[You] let not sin therefore reign in your mortal body" (brackets mine). You have the power to stop the reign of sin in your life, or the Lord would not have given you this command.
A mistaken belief that we can't help but sin is one of the biggest reasons that we do sin. The power of sin (see note 9 at Ro 5:21) has been broken in our lives, and the only reason we Christians sin is because we haven't renewed our minds with the reality of our new lives with Christ (see notes 7 and 8 at Ro 6:6).
Note 5 at Ro 6:12: The word "therefore" makes our ability to end sin's reign in our lives that this verse speaks of, dependent on the truth that was just expressed in Ro 6:11 (see note 3 at that verse). We have to know beyond any doubt that our "old man" is dead and gone; then, and only then, will we be able to renew our minds and end the dictatorship of sin in our lives.