Scared of dying :S Help?

Hey, I'm a 15 year old girl and I worry about death everyday, i go to sleep worried that i'm going to die in my sleep and it makes it harder to enjoy life by worrying about it ending. I don't want to think about something so morbid everyday but i just cant help it. If one of my friends were to say (example) "Hey, next year we have to do this!", I would instantly feel worried that i wouldnt be around for it, that something could happen and I could be gone. What should I do about it? Please no nasty comments :)