Anxiety is a general term for several disorders that cause nervousness, fear, apprehension, and worrying. These disorders affect how we feel and behave, and they can manifest real physical symptoms. Mild anxiety is vague and unsettling, while severe anxiety can be extremely debilitating, having a serious impact on daily life.
People often experience a general state of worry or fear before confronting something challenging such as a test, examination, recital, or interview. These feelings are easily justified and considered normal. Anxiety is considered a problem when symptoms interfere with a person's ability to sleep or otherwise function. Generally speaking, anxiety occurs when a reaction is out of proportion with what might be normally expected in a situation.
Anxiety that is out of proportion with what is normally expected in a situation can be caused by irrational or catastrophic thoughts about the situation. Such as, "My life will be over if I fail this test". Also any thoughts containing absolutes: can't, never, always, etc. For example "I can't relapse again". Anxiety is also caused by the unknown. The less we know about a particular situation or topic the more irrational or catastrophic our thoughts can be.