I have severe anxiety disorder and have fought it for years. I know just how you feel. The first thing to do is see a good doctor and get started on medication, it makes a huge difference. Anxiety disorder is a physical disease and needs to be treated just like any other disease. Then start seeing a good counselor, one you feel you can trust. After that, finding an activity or hobby that makes you feel good is a good way to stop the negative thoughts. Surround yourself with people who care about you, and can make you laugh. Here’s a good book to read: The Anxiety & Phobia Workbook by Edmund J. Bourne, Ph.D.
The thing that most helps me is to remember these two things: Nobody ever died of an anxiety attack. There are millions of other people who have anxiety, I am not alone.
So smile, you’re going to be just fine.