You need to tell him that while you can understand that he is disappointed, that what you really need most is his understanding, rather than being angry with you. That you wish yourself you felt more confident about taking the exam and you did not feel properly prepared and got really scared. That you know he only wants what is best for you, as do you. Then come up with a proactive solution, such as telling him you are going to get a tutor, or ask him if he could help you or that you have even talked with a counselor about the situation. In summary ask for his understanding rather than his anger.
Then you need to find a tutor or do something constructive to show him and yourself (more importantly) that you will take real action to overcome this fear by being prepared.
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