Internal Struggle
When I was younger I heard an old Chinese story about the two dogs (there's also a similar Native American story about two wolves). Basically, two animals are fighting, one represents all that is good while the other all that is evil. Which one wins depends on the one you feed.
Paul illustrates the internal struggle in Romans 7:18-25 "For I know that in me, that is, in my flesh, dwells no good thing. For desire is present with me, but I don't find it doing that which is good. For the good which I desire, I don't do; but the evil which I don't desire, that I practice. But if what I don't desire, that I do, it is no more I that do it, but sin which dwells in me. I find then the law that, while I desire to do good, evil is present. For I delight in God's law after the inward person, but I see a different law in my members, warring against the law of my mind, and bringing me into captivity under the law of sin which is in my members. What a wretched man I am! Who will deliver me out of the body of this death? I thank God through Jesus Christ, our Lord! So then with the mind, I myself serve God's law, but with the flesh, sin's law."
As Christians, we should know that there is a war waging inside of us. Galatians 5:16-24 "But I say, walk by the Spirit, and you won't fulfill the lust of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, that you may not do the things that you desire. But if you are led by the Spirit, you are not under the law. Now the deeds of the flesh are obvious, which are: adultery, sexual immorality, uncleanness, lustfulness, idolatry, sorcery, hatred, strife, jealousies, outbursts of anger, rivalries, divisions, heresies, envy, murders, drunkenness, orgies, and things like these; of which I forewarn you, even as I also forewarned you, that those who practice such things will not inherit God's Kingdom. But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faith,* gentleness, and self-control. Against such things there is no law. Those who belong to Christ have crucified the flesh with its passions and lusts."
So what should we do? ""Repent therefore, and turn again, that your sins may be blotted out, so that there may come times of refreshing from the presence of the Lord," Acts 3:19 Also, "Put to death therefore your members which are on the earth: sexual immorality, uncleanness, depraved passion, evil desire, and covetousness, which is idolatry. For these things' sake the wrath of God comes on the children of disobedience. You also once walked in those, when you lived in them, but now you must put them all away: anger, wrath, malice, slander, and shameful speaking out of your mouth. Don't lie to one another, seeing that you have put off the old man with his doings, and have put on the new man, who is being renewed in knowledge after the image of his Creator, where there can't be Greek and Jew, circumcision and uncircumcision, barbarian, Scythian, bondservant, or free person; but Christ is all, and in all." Colossians 3:5-11