Hi, I'd like to help you with your question.
I want to reassure you that your fear is very common. People fear death at many levels. Some fear how they will die, some fear for the people they will leave behind, and others fear what will happen after they die. Whatever they fear, most people are afraid of death. Humans are intelligent beings that naturally look deeper than the surface for meaning and understanding. When you have something that there is no answer to and you cannot find the meaning, the unknown can scare you. Also, dying can be a fear of the loss of control. No one has control over how they die or when. This triggers fear in a lot of people. Thinking about your fear at night when you are alone is also common.
Understanding that a fear of death is natural is good. However, it sounds as if the death of your uncle and the viewing of his body as triggered a possible phobia for you. Phobias can occur when a fear becomes the focus of your thoughts and starts to interfere in your daily life.
You can begin to deal with your fear by understanding your fear. Read what you can on the topic. One book you can try is called "Be Not Afraid: Overcoming The Fear Of Death" by Johann Christoph Arnold. Another one is "Otherwise Fine: Moving Outside the Frame to Conquer the Fears of Dying" by Susan Barry. A great general phobia book is called "The Anxiety & Phobia Workbook" Edmond Bourne.
Try relaxation techniques. One helpful technique is to lie in bed and starting with your toes, tense each part of your body, tightening your muscles as hard as you can then relax them. Go from your toes to your calves, thighs, stomach, etc. until you reach your scalp and face. Tighten then relax. As you do this, each time you release your muscles, take a deep breath then breathe out. Remember this exercise each time you find yourself thinking about your fears.
You may also want to consider therapy. Cognitive Behavioral Therapy (CBT) is very effective in resolving phobias and fears. A good therapist can diagnose your fear and guide you through and help you learn to relax and refocus your thoughts. They can also help you explore ways to help yourself.
I hope this has helped you,