I totally understand. I do the same stuff. I get some physical symptoms, like a headache, or neck pain and I assume I'm dying. THe thing is, you have to say to yourself...."I'm in control of how I think, and I'm fine". The thing that got me to really realize this, was reading that so many people have anxiety about their health. Being sick with the kind of things I think I'm sick with is really rare. So, the chances of it being in my head are much better than it being something really serious.
Hope this helps.