I've always felt that I don't belong in this world, is this normal?

When I read this post, I was really shocked. This is because I have experienced EVERYTHING you have written. I literally mean EVERYTHING and EVERY SINGLE WORD including the fact that I m almost 18 now. When I read it, I thought as if I myself had written it. Many times I feel like I don't belong to this world...like I am different from others...like I m the only one of my kind. I have always enjoyed writing stories and creating worlds in my mind with me being the main protagonist. I have had difficulties in socializing but now am through it. I love my family and friends dearly.....But I always feel like something is missing....that I don't belong here. Whenever I watch sci-fi movies, I am so much engrossed in them that I wish those were real. Also, have you ever felt that you were born for a purpose? Or there is something very important you and only you can do? Or like something or someone is waiting for you? Like there is something you have to do but you dont know how, when and where....I have had these feelings many times.