Don't be negative. As other people have said: Think of other people who are less fortunate than you are and be thankful for what you have. Your life (for most things) is what you make it. So make your life a happy place
I can't tie shoes. - You can always learn how to. It took me a long time to learn.
I can't swim. - Again, you can learn how to if you want to.
I can't ride a bike. - For a third time, you could have someone help you or just learn on your own.
If the above reasons are some of the worst things about your life, your life isn't that bad :-/
I have scoliosis. -

Sorry.
I am only living right now because of music. - I'm sure there are many things to live for in your life. Suicide is NOT the answer. (just saying

)
I have only one friend that uses me. He can't really be considered a friend, but he's the only person at school I have to talk to. - Make new friends. I'm sure there are a lot of people that would like to be friends with you if they get to know you.
I am only good at math, every thing else I suck at. - Don't be too hard on yourself.
I have terrible vision. - Glasses/contacts/whatever.