This weekend I am preaching on "Sinners and Saints" (the Christian as Sinner/Saint). As a part of my sermon prep (at the end of my process) I went back through a classic that everyone should own. One that I picked up back in '95 that was immensely helpful for me. Thomas Boston's Human Nature in Its Fourfold State is a thorough and practical treatment of human nature before the fall, in our sinful condition, in a state of grace, and in the eternal state. Check out the pastoral angle in Boston's work as he interacts with the Christian who is wrestling with sin and discouragement.
Case 2: "If I were a new creature, sin could not prevail against me as it does."
Answer: Though we must not lay pillows for hypocrites to rest their heads upon, who indulge themselves in their sins... yet, it must be owned "the just man falleth seventy times a day"; and inquity may prevail against the children of God. But if you are groaning under the weight of the body of death, the corruption of your nature; loathing yourself for for the sins of your heart and life; striving to mortify your lusts; fleeing daily to the blood of Christ for pardon, and looking to His Spirit for sanctification: though you may be obliged to say with the psalmist, "Iniquities prevail against me;" yet you may add with him, "As for our transgressions, thou shalt purge them away" (Ps. 65:3).
...Let not therefore the prevailing of corruption make you, in this case, conclude you are none of God's children: but let it humble you, to be the more watchful, and to thirst the more intensely after Jesus Christ, His blood and Spirit; and that very disposition will evidence a principle of grace in you which seeks the destruction of sin that prevails so often against you. (pp. 234, 235)
If you don't have it, you need it. Pick up Thomas Boston's classic - Human Nature in Its Fourfold State