Like everyone else, I have struggled with besetting sins. I am happy to say, thanks to the grace of God in Christ, I have overcome many of them. Yet I am very far from perfect and I struggle daily, as we all will while we are still in the flesh. I have found that the key is surrender.
When we try to be holy in our own strength we inevitably fail, and end up looking to our sin rather than looking to Christ. We need to come to a place where we recognize and confess our weakness, understanding our total dependence upon God for our sanctification. Prayer is essential in all of this, along with constantly feeding upon God's word. We need to always look to Christ and refresh our hearts and minds with the Gospel on a daily basis. Don't keep gazing intently at your sin, confess it and look to Christ!
Make it a habit to continually surrender your will to the Lord, and purpose in your heart not to sin but to do his will, by his grace. You will not attain perfection, you will sin, but you will grow in holiness as God sanctifies you by the power of the Holy Spirit through the renewal of your mind by the Word. Know who you are in Christ, and let the word of God dwell richly within you.
On that note, let me leave off with this wonderful passage from Ephesians, that we may all be edified by the knowledge of who we are in Christ;
"Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, with which he has blessed us in the Beloved. In him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace, which he lavished upon us, in all wisdom and insight making known to us the mystery of his will, according to his purpose, which he set forth in Christ as a plan for the fullness of time, to unite all things in him, things in heaven and things on earth. In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will, so that we who were the first to hope in Christ might be to the praise of his glory. In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory."
- Ephesians 1:3-14 (ESV)