Well, he does have that kind of power, he likes to make the mind his playground by inflicting his ugly thoughts. Where do you think all of the evil and trash that's put out by Madison Ave. and Hollywood comes from? However, guess what "greater is He that is in you than he that's in the world". Your mind can be renewed by the washing with the water of the Word. You can have the mind of Christ. God has not given you a spirit of fear, but of power, love and ~sound mind~.
The devil only has as much power as you give him. Once you know the truth (against his lies and deception) the truth will set you free. Jesus Christ came to undo all of the works of the devil. So there, he is a defeated foe.
It's easy to know when a thought comes from the enemy. If you are filling yourself up with the Word of God, then anything that is vile and contrary is not of God.
I'm glad the site helped you. Do not be discouraged and pursue your freedom from bondages. God is so, so good. Take courage, be strong, the Lord your God is a mighty fortress.