It can be very difficult to master our thoughts. There is very little accountability for what takes place in our brain, at least not until someone invents "thought billboards" conveniently showing our daydreams to the world. Certainly we would be more cautious about what passed through our minds if it was broadcast so clearly to the world!
Fortunately, that hasn't happened yet, and hopefully it never will. However, we are still given the responsibility to govern our thoughts and align them with God's principles. Fortunately, Paul gives us some insight into what God expects in our minds.
Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things. Philippians 4:8
God wants your thoughts to be focused on things that are true, right, and decent. Your mind isn't to be used as the storehouse for all of the hateful things you wish you could say but don't, nor is it to be used to conspire against others or to drool over the attractive person close to you. (Unless its your spouse....then by all means drool away!)
No, our thoughts are to be positive and pleasing to God. We shouldn't think about all of the negative potential of people and situations; that isn't helpful and it pollutes our minds. Instead of filling your mind with thoughts that are negative and unproductive, fill your mind with the goodness that comes from God. Pay careful attention the things you read, listen to, and watch on TV. These probably have a bigger impact on your thoughts than you realize.
Let your thoughts reflect the transformation in your heart and mind that only comes from God.
But God has given us his Spirit. That's why we don't think the same way that the people of this world think. 1 Corinthians 2:12