Satan often torments and troubles Christians by rubbing their past sins in their face. Some Christians are constantly wrestling with guilt and shame for the sins they've committed. Thomas Brooks discusses this in part 4 of
Precious Remedies Against Satan's Devices. He gives six remedies to fight this devious device of Satan. I've listed five of them below, which I've edited for length and readability.
"One device that Satan uses to keep souls in a sad, doubting, and questioning condition – to make their life hell – is to causing them to constantly muse upon their sin, and to mind their sins more than their Savior, yea, so to mind their sins as to forget or even neglect their Savior. Their eyes are so fixed upon their disease, that they cannot see the remedy; they so muse upon their debts, that they don't have mind or heart to think of the One who cancels all debts."
Remedy 1: Weak believers must consider that though Jesus Christ has not freed them from the presence of sin, he has freed them from the damnatory power of sin. The law cannot condemn a believer, for Christ has fulfilled it for him; divine justice cannot condemn him, for Christ has satisfied justice; a believer's sins cannot condemn him, for they in the blood of Christ are pardoned; his own conscience cannot condemn him because Christ has acquitted him (cf. Rom 8:1).
Remedy 2: Consider that though Jesus has not freed you from the molesting and vexing power of sin, he has freed you from the reign and dominion of sin. Remember – it is one thing for sin to molest and trouble you, but it is another thing for sin to reign and have dominion over you (Rom. 6:14). Sin may rebel, but it shall never reign in a saint. Though a Christian may fight with sin, he will never serve it as if it were his master.
Remedy 3: Constantly keep one eye upon the promises of the forgiveness of sin, and the other eye upon the inward operation of sin. Truly God will graciously forgive the sins in his people that he will not fully subdue in them (in this life) (Jer. 33:8; Is. 43:25). In your groaning over sin, cast an eye upon those precious promises of forgiveness. They will bear up and refresh your spirit in the darkest night and under the heaviest burden of sin.
Remedy 4: Look at all your sins as charged upon the account of Christ, as debts which the Lord Jesus has fully satisfied. Not one farthing of the debt that was owed by you is unpaid. All our debts, by his death, Jesus discharged; we are freed, and he is exalted to sit down at the right hand of his Father, which is the height of glory, and the greatest pledge of our blessedness (Is. 53:6, 2 Cor. 5.21). He has fully paid for all our sins.
Remedy 5: The Lord uses this distress, trouble, and vexation that our sin brings us. He uses these burdens to keep us humble and low in our own eyes, and to make us depend upon his help alone, whereby sin may be subdued and mortified. He uses these vexations so that we may learn to rest on Christ more and more; he uses them to detach us from the world, to make us long for heaven more, and to make us love others who are burdened with sin like we are. In his sovereignty, God uses Satan's vexations for our good and his glory.