9/11 impacted so many people in so many ways. Whether you witnessed it in person or on television like most of America, it impacted everyone. Since you tell us you witnessed it first hand you certainly would need to talk with someone about this.
Professional help would be a great way to help you work through he anxiety and PTSD you are trying to cope with. Since your parents are not believing that you need help, it may be a little more difficult to get help. Is there a family friend, a school counselor or pastor that you can talk with?
Working through PTSD can be difficult when you are trying to work through it alone. Talk , journal to help yourself work through this when you feel you need to get some feelings out. It is healthy.
Don't try to do this alone.
We are here.