We understand that you're struggling with a persistent song in your head and feelings of evil and badness. First, let's address the issue of the song. Scripture tells us to take every thought captive to obey Christ (2 Corinthians 10:5 WEB), and this includes persistent songs or any other intrusive thoughts. Pray for God's peace and His thoughts to fill your mind instead. "Finally, brothers, whatever things are true, whatever things are honorable, whatever things are just, whatever things are pure, whatever things are lovely, whatever things are of good report; if there is any virtue, and if there is any praise, think about these things." (Philippians 4:8 WEB).
Regarding your feelings of being evil and bad, we want to remind you that there is no condemnation for those who are in Christ Jesus (Romans 8:1 WEB). It is important to remember that everyone has sinned and fallen short of the glory of God (Romans 3:23 WEB), but through faith in Jesus Christ, we can be saved and made new. If you have accepted Jesus Christ as your Lord and Savior, then you are a new creation and forgiven in His name. If you have sin in your life, it is not because you are evil or bad as a Christian Believer but may need to repent. "If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness." (1 John 1:9 WEB). In Jesus Christ, we are washed clean.
Concerning your request for deliverance from demons, it's important as Christian Believers we cannot be possessed, but we can be oppressed. First, ensure you have truly accepted Jesus Christ as your Savior, and you can pray for God to cast out any evil influence. "Be sober and self-controlled. Be watchful. Your adversary the devil, walks around like a roaring lion, seeking whom he may devour." (1 Peter 5:8 WEB). But "submit yourselves therefore to God. Resist the devil, and he will flee from you." (James 4:7 WEB).
Let's pray together:
Dear Heavenly Father, we come before you in the name of Jesus Christ, lifting up our brother/sister in need. Lord, we ask that you remove this persistent song and fill their mind with your thoughts, your peace and your truth. Cast out any evil influence, Lord, and help them to stand firm against the schemes of the devil. Remind them of their true identity in Christ—that they are forgiven, redeemed, and made new. Help them not to dwell on feelings of evil and badness, but to walk in the light of your love and forgiveness. Cast out any fear except for the reverent fear of the Lord and any condemnation from their life. In Jesus Christ's name, we pray. Amen.
We are thankful you ended your request with In Jesus Christ name.