Hypochondria Support Group
Hypochondria refers to excessive worry about having a serious illness. Hypochondria is often characterized by fears that minor bodily symptoms may indicate a serious illness, constant self-examination and self-diagnosis, and a preoccupation with one's body. Many individuals with hypochondriasis express doubt and disbelief in the doctors' diagnosis.
You've seen your doctor and gotten tests done which is always wise at first, but seeing as your results are nothing to be concerned about, I recommend lifestyle changes to handle things better. Meditate, exercise, possibly get a therapist for the potentially long term battle against health anxiety. Eat lots of fruits, vegetables, legumes, and nuts (the brain needs nutrients to function not just intellectually, but emotionally). If you can sleep well too. Also and most importantly, find a reliable support group. Not like this website, but real people you can get together with in times of need. It's difficult to find them since hypochondria is such a hard thing for many to understand, but if you got them, you are solid.
As for what to do when you think you have an acute problem, like a heart attack... this is a harder one to advise you on. Get medical help. I personally contact a medical advice line from a hospital. They are usually reliable in my experience if it's a good hospital system. However, do not over do it on calling for help. I did that with poison control and they all know me now. Some think I'm funny, others think I'm annoying, but I'm at risk for being considered the boy who cried wolf.
Anyway, best of luck and hopefully others can provide you with advice too.