You keep testing negative because you never had an exposure to HIV. In an adult, the only risks for HIV transmission are unprotected anal or vaginal sex and sharing IV drug needles with infected users. Since you were never exposed to HIV, you keep testing negative. Stop taking tests for a disease that you were not at risk for.
Yes there is help available, you can reach out to a counselor in your neighborhood. A one on one session must help you in overcoming this irrational fear. As stated by Chima7, you were never at a risk from your event.