You didn't catch HIV from antibiotics injection because 3 tests have shown otherwise. The tests are reliable, so all the last 2 did was prove your samples hadn't been switched by mistake the first time.
You have been professionally diagnosed to be HIV free. The only way you will be anxiety free is to accept that instead of worrying about a disease you can't catch. btw, there are many diseases out there that you can catch, so no reason to focus on this one, but you don't worry about the others, so try to think the same way about HIV which you can't even catch.
If you can't rid yourself of these fears then therapy is the route to go. I know it isn't easy to just get rid of fears, but help is available if you are not able to resolve it on your own by accepting the professional diagnosis.