Do I have autism? (15 years old)?

Most people are diagnosed when they are younger unless you have a very mild case. All the signs and symptoms you said you have seem like very minor things that many people have. It seems you have a case of writers block. Many people experience the struggle of not being able to put their thoughts into words. I highly doubt you have any disorder or else it would have been detected when you were much younger. If you really think you have a problem then you should see someone.