"Now one of the things we must cast out first of all is fear. Fear narrows the little entrance of our heart. It shrinks our capacity to love. It freezes up our power to give ourselves. If we were terrified of God as an inexorable judge, we would not confidently await divine mercy, or approach God trustfully in prayer." (Thomas Merton)

There is a natural and instinctive fear, such as fear of a poisonous snake or fear of fire. But most of our fears are unnatural and baseless, except in our mind. Many people live in fear in the present moment because of an event that happened in the past which still controls them. They are afraid to trust God because something that happened in the past, which they blame God for. But this fear paralyzes one from the gift that is most important, namely love. 

What can one do? First, identify the fear. This kind of fear could be the devil's way of keeping you away from God. Second, what is the source of the fear? Third, ask God for the grace to choose to address the fear by doing the opposite. Fourth, keep doing this until it becomes a habit. Fear doesn't go away quickly, but as you continue to attack it, it will be less dominating in one's life.