Albert Barnes on Romans 7:19

For the good, etc. This is substantially a repetition of what is said in Rom 7:15. The repetition shows how full the mind of the apostle was of the subject; and how much inclined he was to dwell upon it, and to place it in every variety of form. It is not uncommon for Paul thus to express his intense interest in a subject, by placing it in a great variety of aspects, even at the hazard of much repetition.

(u) "the good" Gal 5:17