Paul did not say the sin nature existed in him. If you read all of Romans 7 you see the context is his body. In saying 'that is in my flesh' he explains what he means by sin that is in me. Romans 8 carries on the same theme of Paul advising not to follow the deeds of the body. Romans 8 says Christians are waiting for only one thing - the redemption of the body. All the best for the season.