Your symptoms don't suggest STD; I know of no STD that causes prolonged testicular aching. The negative tests can be considered reliable, especially twice by knowlegeable providers like a UK GUM clinic. I have often said on this forum that when a person suspects his or her own symptoms are due to anxiety, they usually are right. You can initiate a new sexual relationship without fear of infecting your partner.
Good luck-- HHH, MD
Ben
HHH, MD