I'm very young but I have this horrible intense fear of Death/Dying, what do I do?

I have this terrible fear of death, almost to the point where its inhibiting me from enjoying my life, I'm always thinking about getting sick somehow or getting old and being close to death. I fear it so much that I avoid anything that I think might make me have a shorter life-span. I'm a total health freak, I don't eat your average food. I eat as healthy as knowingly possible, I work out a lot, I take all kinds of vitamin/mineral/dietary supplements, I never drink or smoke or do drugs, well except for wine. But I still know that no matter what I do I will most likely get old and die someday and it makes me so depressed. I don't believe in heaven or any fantastical "after-life" because it just doesn't seem logical or scientifically possible to me. Its just a fairy-tale to ease peoples' fears. I'm a college student and a biochemistry major and I only base what I believe on what I can actually see and what can be scientifically proven, I absolutely can't make myself believe anything else. I honestly don't think there's anything after death...I think death is just like when you sleep but you don't dream, nothing else. Just black nothingness, that's it. What should I do? Its starting to make me paranoid and depressed. I need advice. I want to live to be the oldest person in the world because I hate the idea of dying more than anything, but I am so afraid that I won't live that long for whatever reason.