What the heck is wrong with me?

I have a huge house, lots of space, horses, a cat, a dog, a guitar, riding lessons + guitar lessons, flatscreens all over the house, a laptop, and I'm not happy. I wake up every morning annoyed, if I can even get to bed after tossing around for an hour before I fall asleep and waking up in the middle of the night. I cry at least once every day, sometimes completely spontaneously. I am very irritable, yelling at people who don't deserve it. I feel I have no future and no purpose. I've never seriously considered suicide because death scares me. Things I once had interest in and was doing vigorously I now despise or have lost interest. I am the top of the class yet I always feel like I'm an idiot. I don't know what's wrong with me and I don't know what to do. I'm crying as I write this because what I want is so simple for most people- happiness. Why do I have everything but the one thing I need? Selfish, yes, I know. But I really need help. Thank you so much.