Are you afraid of...?

At your middle school/high school, there should be a counselor and you should go talk to that person. Don't be embarrassed, I'm 16 and went through the exact same thing at your age. You're starting to grow up and understanding what the world is and how it works. You said it has happened since you were little. It could be a bad experience (relative dying etc...) that's causing you to feel this way. I think most people have a fear of the unknown and everyone has a fear of dying at one point or another. You need to realise that there is NOTHING you can do, and that eventually, everyone does die. However, with all the new medical advancements coming out, you could honestly live to be a couple thousand years old, if not older (read up about it, they tested it on rats and it is promising). The best thing to do is follow your heart and do what you want to do. Live your life and do what you want to do, never let anyone else tell you you can't do something. And at the end of the day, when the time finally does come, you can look back and be proud of the life you lived. Is there an after life? I don't know. But if there is, it's just one other adventure to look forward to. Hope I helped ;)