Well, one thing is true, you don't have hiv. That is clear from all of your negative tests. The only thing keeping the thought in your mind is that you fear that it is wrong. The way to stop the thoughts is to get rid of the fear and to do that you need to embrace the fact that you could have hiv. It seems wrong but the less value you give a thought, the less you feed it with argument the less of a hold and meaning it will have. Whenever you get a though saying 'what if' or 'but what about this' whatever it is that comes to mind, just say to yourself, 'yeah ok whatever', or 'yes i have hiv'. Keep doing it until you start to feel less worried by the thoughts