I just can't mentally cope with the fact that I think I have a sexually transmitted disease. How would you deal with this?

I have had all the tests for sexually transmitted diseases and they were all negative. I also waited 3 months and repeated it again a year later and everything was negative again. However, a year ago I had strange symptoms that are still bothering me psychologically. I have also found other people on the internet who have had similar experiences to me and some of them have apparently gotten diarrhea that won't stop. I am really racking my brains as to whether I will soon get symptoms that will not stop. I know that luckily many people will be more sensible than me and will take better care of themselves. I haven't slept with anyone for over a year because it's all bothering me. I'm scared that it will soon happen and that I won't feel healthy anymore. Please take it seriously. I understand if people think I'm a hypochondriac but unfortunately that doesn't help me. It just drags me down even more because I don't feel understood. I don't think it's just psychological. For example, I lost 12 kilos in just one month. I had diarrhea, rashes, feeling unwell and scaly skin on my face even my colleagues said they thought I was ill even though I tried not to let it show I got the symptoms 4 days after my last intercourse I thought it was HIV but the test was negative like the others unfortunately many people don't believe that there could be other diseases that we don't know about yet but there are things that make it not so unrealistic for example only 11,000 viruses are known but according to calculations there are 320,000 I wish a doctor could help me if I'm right and get seriously ill but unfortunately I don't think anyone will be able to help me because apparently nobody knows how would you deal with it if you were in a similar situation perhaps you could give me some encouragement that would be really great