How do you force the dark thoughts of death away?

I know a couple of people with terminal cancer, one is a male in his 30's with a young son, the other is a lady closer in age to my parents. I lay awake last night thinking of them both, and their families, and how on earth all of them could come to terms with a diagnosis like that. It made me think that everyone I know is going to die. I'm going to die, and even if we make our time and our relationships worthwhile while I'm here, how will I ever cope when I lose those close to me? How does anyone cope? I'm not suicidal but it makes me think maybe it's easier for me to go first so I would never have to live through the pain losing someone I love. The dark thoughts, about death, about loss, have escaped their keepers in my mind and are running rampant. How do I make it stop?