What is wrong with me?

Please help me. I have nothing to be depressed about at all. I have a wonderful girlfriend, a great group of friends, a secure life and a good future and past. But I am very depressed. I can't seem to enjoy anything because I keep thinking that it's going to be over shortly, and the next thing I do will be over, and on and on until I die. And I'll eventually lose my gf because of this because one of us will die. And I'll have to watch her grow old and I just want things to be the same as they have been. I've been upsetting her because I'm so down, and she has enough trouble without that. This has been going on for about 3 weeks nearly and I hate it, but I can't get it out of my mind. I've lost my appetite and I had a panic attack, where I had to call an ambulance. Please help, I can't cope like this.