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Made Holy by the Spirit
The Lord doesn't leave the people he justifies in the slavery from which he saved them.
Sanctification is God's work in believers, renewing the whole person after his image and enabling them to die more and more to sin and live to righteousness (WSC 35 (opens in a new tab)). The Holy Spirit applies the death and resurrection of Christ to those who belong to him. Scripture calls them to put off the old man and put on the new (Ephesians 4:22-24 (opens in a new tab)).
We must be careful here. Justification is God's once-for-all declaration that a sinner is righteous in Christ. Sanctification is the Spirit's ongoing work within that justified sinner. We can't earn justification by becoming holy, but those whom God justifies he also sanctifies (WCF 11 (opens in a new tab) and 13). Neither doctrine should swallow the other.
Why Is Sin Still in Me?
Because sanctification is real but incomplete in this life. Westminster Confession 13.2 says corruption remains in every part of a believer, and from that comes a continuing war between flesh and Spirit. Paul describes that war in Galatians 5:17 (opens in a new tab). A mature Christian may feel it more keenly as he learns how holy God's law is.
Remaining sin gives no permission to stop fighting. Romans 6 (opens in a new tab) says the dominion of sin has been broken. Your old master has no right to rule you. Yet its habits and desires still trouble you. You need the Spirit's strength, and you must use the means he gives.
Don't measure holiness by how many dramatic experiences you've had. A man may speak loudly about victory and still refuse to forgive his family, tell the truth, or keep his promises. Scripture calls us to put sin to death in ordinary conduct, including our speech, labor, relationships, and worship (Colossians 3:5-17 (opens in a new tab)).
How Does God Grow His People?
God ordinarily uses his Word, the sacraments, and prayer (WSC 88 (opens in a new tab)). Sit under faithful preaching, not only when you feel eager to hear. Read the passage in context and pray over what it commands. Worship with your congregation. Where you need correction, receive it with humility.
Put away what makes provision for your sin. If your phone, a friendship, or an hour of the night keeps drawing you back, change your habits (Romans 13:14 (opens in a new tab)). Speak plainly with a mature believer or an elder who can help you. Confess what you've done and seek to repair the wrong you caused. These acts don't purchase God's favor. They are part of the obedience his grace produces.
Don't treat falling again as proof that you should stay away from Christ. Return to him. Nor should you call a planned return to sin a harmless stumble. If you've decided to keep a sin and refuse correction, you need repentance, not reassurance. Read Struggling With Sin for more detailed help.
Will This Fight Ever End?
Yes. Not because you will finally become strong enough to live without Christ, but because he will finish what he began. The Confession teaches that the sanctified part overcomes through the continual supply of strength from Christ's sanctifying Spirit (WCF 13.3 (opens in a new tab)). At death the souls of believers are made perfect in holiness, and at the resurrection their bodies will share that glory (WCF 32.1-3 (opens in a new tab)).
Until then, confess, believe, pray, obey, and keep coming to Christ. He is the one who saves you and who teaches you to walk. The Christian life isn't a wager that you'll improve enough to be accepted. It is the life of a person whom God has received in his Son and is remaking by his Spirit.