Thankfully, the Purtians were full of spiritual wisdom on practical Christianity. In the article below, we learn the distinction between increasing in holiness because we're increasing in grace and increasing in outward righteousness because we're obsessed with the law (instead of the lawgiver). I hope it proves edifying to you as well.
The Difference Between Legal and Gospel Mortification
By Ralph Erskine
Mr Ralph Erskine (1685-1752) was the son of a Covenanter, a Scottish Presbyterian minister who was imprisoned for field-preaching and refusing to countenance the official episcopalian church. A younger brother of the famous preacher Ebenezer Erskine, Ralph was an evangelical pastor with a love for the truth of God’s Word and the doctrines of the Lord Jesus Christ. He is still well known for his “gospel sonnets”, a collection of evangelical poems intended to press home particular Gospel truths. The Erskine brothers were “Marrow Men”, supporting Mr Thomas Boston’s reprinting of the “Marrow of Modern Divinity”, a Puritan book which distinguishes the Covenant of Works from the Covenant of Grace.
1. Gospel and legal mortification differ in their principles from which they proceed. Gospel mortification is from gospel principles, viz. the Spirit of God [Rom. 8. 13], 'If ye through the Spirit mortify the deeds of the body, ye shall live'; Faith in Christ [Acts 15. 9], 'Purifying their hearts by faith'; The love of Christ constraining [2 Cor. 5. 14], 'The love of Christ constraineth us.' But legal mortification is from legal principles such as, from the applause and praise of men, as in the Pharisees; from pride of self-righteousness, as in Paul before his conversion; from the fear of hell; from a natural conscience; from the example of others; from some common motions of the Spirit; and many times from the power of sin itself, while one sin is set up to wrestle with another, as when sensuality and self-righteousness wrestle with one another. The man, perhaps, will not drink and swear. Why? Because he is setting up and establishing a righteousness of his own, whereby to obtain the favour of God here is but one sin wrestling with another.
2. They differ in their weapons with which they fight against sin. The gospel believer fights with grace's weapons, namely, the blood of Christ, the word of God, the promises of the covenant, and the virtue of Christ's death and cross [Gal. 6. 14] 'God forbid that I should glory, save in the cross of the Lord Jesus Christ, by whom [or, as it may be read, 'whereby,' viz. by the cross of Christ,] the world is crucified to me, and I to the world.' But now the man under the law fights against sin by the promises and threatenings of the law; by its promises, saying, I will obtain life; and win to heaven, I hope, if I do so and so; by its threatenings, saying, I will go to hell and be damned, if I do not so and so. Sometimes he fights with the weapons of his own vows and resolutions, which are his strong tower, to which he runs and thinks himself safe.
3. They differ in the object of their mortification. They both, indeed, seek to mortify sin, but the legalist's quarrel is more especially with the sins of his conversation, whereas the true believer should desire to fight as the Syrians got orders, that is, neither against great nor small, so much as against the King himself, even against original corruption. A body of sin and death troubles him more than any other sin in the world; 'O wretched man that I am! who shall deliver me from this body of death?' [Rom. 7. 24]. His great exercise is to have the seed of the woman to bruise this head of the serpent.
4. They differ in the reasons of the contest. The believer, whom grace teaches to deny all ungodliness, he fights against sin because it dishonours God, opposes Christ, grieves the Spirit, and separates between his Lord and him; but the legalist fights against sin, because it breaks his peace, and troubles his conscience, and hurts him, by bringing wrath and judgment on him. As children will not play in the dust, not because it sullies their clothes, but flies into their eyes, and hurts them, so the legalist will not meddle with sin, not because it sullies the perfections of God, and defiles their souls, but only because it hurts them. I deny not, but there is too much of this legal temper even amongst the godly.
5. They differ in their motives and ends. The believer will not serve sin, because he is alive to God, and dead to sin [Rom. 6. 6]. The legalist forsakes sin, not because he is alive, but that he may live. The believer mortifies sin, because God loves him; but the legalist, that God may love him. The believer mortifies, because God is pacified towards him; the legalist mortifies, that he may pacify God by his mortification. He may go a great length, but it is still that he may have whereof to glory, making his own doing all the foundation of his hope and comfort.
6. They differ in the nature of their mortification. The legalist does not oppose sin violently, seeking the utter destruction of it. If he can get sin put down, he does not seek it to be thrust out; but the believer, having a nature and principle contrary to sin, he seeks not only to have it weakened, but extirpated. The quarrel is irreconcileable; no terms of accommodation or agreement; no league with sin is allowed, as it is with hypocrites.
7. They differ in the extent of the warfare, not only objectively, the believer hating every false way; but also subjectively, all the faculties of the believer's soul, the whole regenerate part being against sin. It is not so with the hypocrite or legalist; for as he spares some sin or other, so his opposition to sin is only seated in his conscience; his light and conscience oppose such a thing, while his heart approves of it. There is an extent also as to time; the legalist's opposition to sin is of a short duration, but in the believer it is to the end; grace and corruption still opposing one another.
8. They differ in the success. There is no believer, but as he fights against sin, so first or last he prevails, though not always to his discerning; and though he lose many battles, yet he gains the war. But the legalist, for all the work he makes, yet he never truly comes speed; though he cut off some actual sin, yet the corrupt nature is never changed; he never gets a new heart; the iron sinew in his neck, which opposes God, is never broken; and when he gets one sin mortified, sometimes another and more dangerous sin lifts up the head. Hence all the sins and pollutions that ever the Pharisees forsook, and all the good duties that ever they performed, made them but more proud, and strengthened their unbelieving prejudices against Christ, which was the greater and more dangerous sin.
Thus you may see the difference between legal and gospel mortification, and try yourselves thereby.
 Mortification: putting to death (of sin)