For those doubting their regeneration -- Case 2.

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, and make the doctrine of God's grace subservient to their lusts, lying down contentedly in the bond of iniquity like men that are fond of golden chains; yet it must be owned, "the just man falls seven times a day;" and iniquity 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 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 you shall purge them away," Psalm 65:3. The new creature does not yet possess the house alone: it dwells by the side of an ill neighbor, namely, remaining corruption, the relics of depraved nature. They struggle together for the mastery. "The flesh lusts against the spirit, and the spirit against the flesh," Gal. 5:17. And sometimes corruption prevails, bringing the child of God into captivity to the law of sin, Rom. 7:23. 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.

--Thomas Boston, from Human Nature in Its Fourfold State.