@ Britt, there is absolutely no reason for you to apologize! Everyone has their problems.
You shouldn't say you hate yourself. And about being very shy, I understand it can be very troublesome. A friend of mine is also very VERY shy. I would say you should just be more open, but that's easy for me to say. I can't really understand, sorry.
And being 18 isn't all that great. It's the age when you need to be able to take responsibility for everything you do. Just enjoy being 15, there will be a time where you wish you are 15 again. But when you turn 18 you might think entirely different about everything.
I agree with you on life, it is, but that's why you hardly ever get bored with life. And it's odd, but that is what is makes it so great too!
Complaining my bum, you say and realize you do have a nice life, but just want something o change, I doubt anyone can say they think their life is perfect, 24/7.
I really hope you will travel around the world (you should go to Austria and Italy to, very very beautifull). And don't worry about being unhappy, as stupid as it sounds, it kind of is the age. And by the way, there is nothing wrong with wanting to be alone. Just don't make yourself lonely.
Wish you all the best wishes!