God has not given us a spirit of fear, but one of power, love, and a sound mind.
Paul tells us in Romans not to be conformed to this world but to be transformed by the renewal of our minds. In Corinthians, he tells us to take every thought captive and to subject them to the knowledge of God. We are told to nail the decree against us to the cross. The Bible gives us knowledge of God. We have precious promises. We have been given gifts, strengths, authority, and righteousness in Christ. We use the weapons of our warfare and are told to seek God's rest. The rest of faith is how we fight. We fight by resisting the devil, the flesh, the world. We fight by resting. The world, the devil, and our own history have filled our minds with lies. But the Holy Spirit is the Spirit of Truth and he has given us everything necessary for life and godliness. We are seated with him in heavenly places and Christ has given us authority and power. The devil, of course, doesn't want us to know or trust in the Word sowed into our hearts. God has given us power through his precious promises. This is why we read our Bible. Each promise and Bible verse that we believe in is a living, active, powerful weapon.
There are many lying thoughts that lie against the truth. Lies like: God doesn't hear me when I pray. God doesn't love me. God isn't helping me. I am powerless. The Bible is not true.
So, how do we take our thoughts captive?
Example: if we are attacked by a thought that we are old and probably will get sickly and die at age 70, or if we are attacked by thoughts that we are losers and we never overcome anything, we have to speak.
How do we speak?
We take the negative thought captive and we use the word of God spoken from our lips (and believed in our hearts) to declare the thought is false.
Example: In the name of Jesus, I declare the thought that I will die at age 70 because such and such runs in my family is a lie. I take this thought captive to the truth of the word which says with long life God will satisfy me and show me His salvation. It is written in Gods word that The Lord is my healer. I choose to believe God. God does not lie.
Another example: In the name of Jesus, I take the thought captive that I am powerless against my sin, flaw, habit, etc. I take that thought captive now and I choose to destroy it by making it subject to Christ. All my sin and the power of sin was nailed to the cross through the victorious death of Christ. The grace of God has appeared which teaches and empowers us to conquer sin. In Christ I have victory over this thought, habit, sin, etc. I choose to believe God's word.
We can also command the thoughts to go. We have the authority to command them to go. But do it in faith.
And always, always pray and hope. Trust that something good is happening.