I will be very direct.

While we are told in God's Word that in this present spiritual battle we call life we wrestle "against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places" (Ephesians 6:12), the true believer is not to "wrestle" with sin. The true believer certainly recognizes the fact that sin is still present in his flesh and understands, in that, that the sin nature is still alive and functioning even after he, himself, has been regenerated and given eternal life, the sinless life of Christ. But the truly born again believer "does not go on sinning" (1 John 5:18a, ESV).

To "wrestle" with sin suggests an entangling involvement with sin. To “wrestle with sin,” then, is just another way of describing willful sinning. In other words, the one who is "wrestling" with sin is actively sinning, regardless of what rationalization is given to suggest otherwise.

To “wrestle with sin” is entirely different from acknowledging the presence of sin, i.e., the fact that the flesh can be tempted. Paul recognized the presence of sin in all who are not yet glorified, which is to say all men, believers and unbelievers alike. He described this graphically in Romans 7. But though he understood and acknowledged the plain fact that his flesh, like everyone else's, desired to sin, i.e., it served "the law of sin," he, himself, the “new creature in Christ” (2 Corinthians 5:17), served "the law of God" (Romans 7:25). In that, he did not actively, i.e., willfully, participate in sin.

The Holy Spirit's plain command is "Walk in the Spirit, and ye shall not fulfill the lust of the flesh" (Galatians 5:16). The person, then, who is actively sinning is making a willful choice to walk in the corruption of the flesh rather than the strength of the Spirit. He is choosing to sin when he has been given the ability to not sin. This is an act of overt rebellion against the rule of God in his life. It is a conscious act which will be judged by the Lord.

1 Corinthians 3:17 If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are. 

This plain warning was given after the cross, and after the resurrection and ascension of Christ to heaven where He now intercedes for all true believers. And it is given by the Holy Spirit through the apostle of the gospel of the grace of God, Paul, who also wrote

Titus 2:11-15 For the grace of God that bringeth salvation hath appeared to all men, 12 Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world; 13 Looking for that blessed hope, and the glorious appearing of the great God and our Saviour Jesus Christ; 14 Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works. 15 These things speak, and exhort, and rebuke with all authority. Let no man despise thee. 

and

1 Timothy 6:3-5 If any man teach otherwise, and consent not to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which is according to godliness; 4 He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings, 5 Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself. 

In other words, God's grace does not change His holiness and its demands and expectations. He has not only paid the penalty for the believer's sin, but He has also given him the power to overcome sin. Not only so, He has provided the believer with very plain instructions as to how he is to gain the victory over sin.

Ephesians 4:17-25 This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the vanity of their mind, 18 Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart: 19 Who being past feeling have given themselves over unto lasciviousness, to work all uncleanness with greediness. 20 But ye have not so learned Christ; 21 If so be that ye have heard him, and have been taught by him, as the truth is in Jesus: 22 That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts; 23 And be renewed in the spirit of your mind; 24 And that ye put on the new man, which after God is created in righteousness and true holiness. 25 Wherefore putting away lying, speak every man truth with his neighbour: for we are members one of another. 

Colossians 3:5-6 Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry: 6 For which things' sake the wrath of God cometh on the children of disobedience: 

Will believers sin? Yes. We are not yet glorified, not yet perfect. The Lord understands our weakness, that we may stumble, and has provided grace for even this.

1 John 1:8-9 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 

Do you see the whole purpose in God's grace? When the believer stumbles into sin, God's forgiveness does two things: (1) it restores him to blessed fellowship with God, and (2) it cleanses him from “all unrighteousness,” i.e., the ongoing contamination of the truly confessed and forsaken sin. So, God’s forgiveness, His grace, is not given to encourage us in our sin, but to empower us to stop sinning.

The believer, then, who is living in willful sin, i.e., is habitually sinning, is not "wrestling" with sin. Rather, he is succumbing to sin. He is walking in willful, rebellious disobedience. To be blunt, he is choosing to sin for his own lustful pleasure instead of obeying the Lord in the power God has given to him to have victory over sin. He is choosing defeat and its losses instead of righteousness and its gains.

In summary, the believer is to "mortify," i.e., to slay, sin whenever its temptations are manifested in his life. He is not to feed it. He is not to continue to breathe life into it. Anything less than an active, that is to say, a willful slaying of the deeds of the flesh will bring, at the very least, spiritual death, for "God is not mocked: for whatsoever a man soweth, that shall he also reap. For he that soweth to his flesh shall of the flesh reap corruption" (Galatians 6:7-8). 

     Flee fornication. (1 Corinthians 6:18a)

     Resist the devil, and he will flee from you. (James 4:7b)

There is no "wrestling" implied in either of these commands, is there! Victory is simply a willful choice to not sin, to not fear, to not succumb to any of our enemy's devices and temptations. The blessings of these very plain and simple instructions can be secured by anyone who truly desires to have victory over sin.