Im empty. Please help?

I dont know why. Sometimes im on top of the world. Im well liked, good at sport, fit, fairly smart. I got through school with great marks and no effort. I live in a good home with good parents. But sometimes i just dont feel like living anymore. One of my friends died and i miss him a lot. I dont look forward to the future.. i just worry about it. I cant stop thinking about the big questions. If im not playing sport i just feel so tired all the time and just want to lie down. I procrastinate a lot. I just dont know what to do. And i just feel guilty about it because theres so many people with such shitty lives who can deal with it, and ive got what anyone would call a great life and i still dont want to live it. I dont even know who i am. Im everything. Im shy and outgoing, i hate going out and i love going out, im an idiot and an intellectual. Im going to die alone. I dont know what to do. I cant talk to anyone because theyll just be like "youve got nothing to complain about". I miss being a kid. 18 years has gone by in a flash. And i feel like ive lived for too long already. I dont know what to do