Nervousness, fears and phobias
Fears can limit our possibilities in life a lot. When afraid, we tend to avoid, whether small things or big.
We can be afraid of many things, like of the dentist, of going into open places, fear of hights. Fear of flying, or of spiders.
Often, fears can be made less intense fairly well by processing the emotions.
A phobia is basically a diagnosis of a particular fear that is so bad, that it makes life really difficult. One can have panic attacks, when confronted with the particular fearful situation.
When there is no psychological cause
Sometimes, fears are caused by brain damage or neurological problems. Processing the fears then won't work well, and other treatment is needed.