Anxiety can show itself as health anxiety. I know, I've had it. You get really fixated on something health wise and are just SURE it's a problem. The reason I bring this up as you have tested and tests are negative. Inability to accept that is anxiety. Anxiety is treatable and I'd see a therapist about this. Really, it is your best way to overcome it. Guilt is powerful as well . But many feel guilty and don't continue to obsess over something like this. However, maybe it is easier than dealing with the emotions of our actions. Again, a therapist is usually excellent at working through that. We are allowed to make mistakes in life. It's human. Speak to a professional so you can get on with life.
You don't list the details, though I'm guessing you posted on one of the HIV forums and were told there was no risk, your doc told you there was no risk, but again, you don't say here what it is you did but since you now know whatever it was had no risk and you're still obsessing over it, you probably have a common phobia about getting HIV. A lot of that can be dealt with by studying thoroughly exactly how one gets HIV so you're informed, because if you had a no risk event and were concerned than obviously you don't have a thorough education about how to protect yourself from HIV. So do that and it should take some of the fear away. When you mention guilt, that implies you either cheated on someone when you did whatever it is your did or you have religious upbringing that has taught you what you did is wrong. If you cheated on someone, you should feel guilty. That's one of nature's ways of letting us know when we did something we're not altogether sure was the right thing to do and teaches us not to do that again if it is something not to do. But letting guilt run away with your life doesn't help you or the person you cheated on. Not doing it again does. If it's religious, well, that's up to you to decide. For centuries now various religions have turned sex into something forbidden, and that's a hard thing to get away from. If you decide that's not what you believe, and what you did didn't harm anyone, then have faith in yourself and set your own path. If you believe in the religious path, follow that and again, if that isn't harming anyone then have faith in that path. Either way, beating yourself up about it for a long time won't help anyone. Learning from it will. If, however, it's not really either guilt or the true possibility of an infection, then it's a phobia, an irrational fear, and if that fear is your only phobia, you need to talk yourself out of it. Not easy to do, but it can be done. If you can't do that, therapy can help you do that. If you have a lot of phobias, not just this one, then therapy is even more important. Peace.