Extremely worried about HIV after I was exposed to it. When is it safe to stop testing and accept negative test results? 3mths? 6mths? 1yr?

HIV testing... Negative testing 6 months from exposure is highly accurate and no further testing is needed as long as there have not been other risky sexual encounters since that time. You can be confident that testing 6 months after exposure to the virus provides highly accurate results. In future avoid high risk sexual encounters, use protection, and get regular screening for HIV/STDs if remain sexually active.
HIV tests. at 3 months then again at 4-6 months should help clarify for contraction of HIV. Either way that is how you will know for sure.