I am using a technicality. Our salvation is dependent upon Jesus Christ and His sacrifice paying for our sins. Saved by grace through faith. Right? So regardless of whether or not I overcome a particular sin in my life at some point in time is not the moment I am officially saved. Sanctification is only a possibility and it necessarily happens after conversion but is not the means unto salvation. That, salvation, is simply by grace through faith in Jesus Christ.
There is no contradiction. Sin will not dominate our lives and no matter how much sin we rack up in our entire lives His grace is sufficient through God's Son's blood. None of us stop sinning in this life, no one is perfect. Therefore we all have sin. My point, however, is that the overcoming of sin is not the means to salvation (the sanctification process), but that is simply found in Christ (justification). This is not to neglect sanctification, mind you. Only that sanctification is a fruit of conversion (don't put the cart before the horse).
Notice that the apostle Paul says to not use our liberty to entertain the lusts of the flesh? Right? How could he say that if it were not so? That sounds extreme, but the reality is that we are born anew. So the statement is true, that no matter how much sin we have His grace is sufficient.
However, and also true, sin shall not dominate our lives. There is no contradiction, His grace is sufficient in that the blood of Christ cleanses us from all sin and also too because of His grace sin shall not dominate our lives. Both sinners and saints sin, the only different between them and us is that we have Jesus. The amount of sin is decreased in our lives compared to the unbeliever, and yet we still accumulate sin in our lives (over time, in totality). The important fact being, Jesus took care of sin at the cross and His grace is sufficient.