We understand that you're going through a troubling time, fearing the unforgivable sin and experiencing unwanted thoughts. First, let's address your concern about the unforgivable sin. In the World English Bible (WEB), Jesus speaks about this in Matthew 12:31-32: "Every sin and blasphemy will be forgiven unto men, but the blasphemy against the Spirit will not be forgiven unto men. Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, neither in this age, nor in that which is to come."
The unforgivable sin is blasphemy against the Holy Spirit. This involves attributing the work of the Holy Spirit to Satan, and it's a deliberate and persistent rejection of God's work in one's life. Based on your message, it doesn't seem that you have committed this sin, as you're turning to Jesus for help.
Now, let's address the unwanted thoughts. In 2 Corinthians 10:5, the WEB tells us: "casting down imaginations, and every high thing that is exalted against the knowledge of God, and bringing every thought into captivity to the obedience of Christ." To deal with these thoughts, first acknowledge that they are not from God. Then, cast them down and replace them with thoughts that align with God's will.
Let's pray together:
"Dear Heavenly Father, we come before you in the name of Jesus Christ to lift up our brother/sister who is experiencing fear and unwanted thoughts. Lord, we know that you have not given us a spirit of fear, but of power, love, and self-control. We cast down any imagination or thought that exalts itself against the knowledge of God. We bring every thought into captivity to the obedience of Christ. Lord, fill our brother's/sister's mind with your peace, your truth, and your love. In the name of Jesus Christ, we pray. Amen."
Remember, God is faithful and will not allow you to be tempted beyond what you can bear (1 Corinthians 10:13, WEB). Stand firm in your faith and resist the devil (1 Peter 5:9, WEB). Whenever you experience these thoughts, turn to God in prayer and trust in His faithfulness. Keep pursuing a right relationship with God. If you find that you would find further benefit from casting off demonic forces within yourself, then pray for God to lead you to a further understanding of your need, the applicable biblical verses on this matter and how to do this now or later in your journey.