Fear is an instinctive behaviour of defending or escaping for survival in the face of danger, usually in response to sudden stimuli, sudden strangers, toxic species, large carnivores and so on. It is a crucial ability for surviving. Regarding what the brain undergoes in a fearful situation, it is hard to say in a few words. The amygdala plays a vital role in the fear system in the human body. You can search for relevant information about fear upon the amygdala. However, the amygdala is not the core of the brain. Please refer to the article, https://www.psychologytoday.com/us/blog/i-got-mind-tell-you/201508/the-amygdala-is-not-the-brains-fear-center. In the article, the author clarifies relationships between fear and threat.
Fear is one of the psychic complex mode in the form of the fear .Fear has a psychic trend & it has in the said line a connection with our brain part in the form of psychological complex which keeps our mind more sensitive with thoughts & fears .
As feel is a psychic problem , for caring out a looking activities you should keep in mind with the firm practice to sit for 15 to 20 minutes silently & if any thoughts prevailed in your mind you should allow the same as a passing phase without thinking & to carry out your routine progress this practice will help us to keep our mind calm & quite & in this atmosphere try to take recourse of power of prayer & meditation