how do I get over my paranoia?

Originally I put this in the STD thread, but I kind of think that it might belong over in mental health...

Over three months ago I had an unprotected sexual encounter with a friend who had only had one sexual partner before me(it was my 1st sexual experience of any kind). Since that time I have been very paranoid and guilty. I got tested at three weeks because they said that was the earliest I could be tested for herpes(I've had an almost irrational fear of herpes since a close friend of mine contracted it several years ago). My results were negative. I have been back about four times since because I was afraid something was an std. I have been to two different clinics and seen multiple doctors. They have all told me that it is paranoia and I have nothing to worry about. A few weeks ago(at the three month period) I went back and got another blood test. It came back negative.

Still, when anything goes wrong or I have an infected ingrown hair(I picked at it like crazy the other day) I am convinced that I have herpes. I feel really guilty about what happened with my friend. How can I stop my paranoia? Also, I don't really have a reason to believe that my friend was ever exposed to herpes(of course, nothing is for certain, I know that). I guess my question is more like.... has anyone else experienced paranoia like this? And if so how did you get over it? I don't know why this is driving me crazy.