Anxiety is the state of being concerned or troubled with distressing thoughts of real or imagined problems. Anxiety is the carnal nature expressing its belief in the inability of God to handle the situation.
These problems exist because a person relies upon there own strength rather than upon God and His Word.
It is the result of leaning upon your own understanding in dealing with your problems.
The Bibles answer to this is, "Cast all your anxieties on Him for He cares about you." (1st Peter 5:7)