Now remember that Paul is a Christian. He is a believer in Jesus Christ. But he is also describing that the sin nature remains within him, despite his Christian status.
Just because a person has believed in Christ and is therefore saved, does not mean the sin nature has become somehow nonexistent within the body.
Christians are just as easily susceptible to sin after salvation, as they were before salvation. Your volition decides which way you choose to lean.
The sin nature leaves an inherent weakness within the individual, that is impossible to overcome, on ones own.
But this should also make it clear, that self-perfection within the individual is also an impossibility.
You cannot eat the right things, you cannot exercise the right way, you cannot meditate in some certain way, as a means to ascending yourself from imperfection to perfection.
There is only one way to gain perfection in ones life, and that begins with faith in Christ for salvation, and learning Bible doctrine for spiritual muscle.
Study [by instruction],
to show thyself approved [spiritually mature]
a workman [student]
that need not be ashamed [ignorant],
rightly dividing [learning, understanding, discerning]
the word of truth [Bible doctrine].