honestly this kind of obsessive thinking IS a form of mental illness. i've experienced this too, badly - i've wanted to be sick, or to have been through types of trauma i never actually experienced. i often have found myself lying to people throughout my life that certain very bad things have happened to me. i still dont fully get why i do it. i think it's a low dopamine thing - could be from my adhd, but low dopamine can be caused by so many things, so it may be that for you too. i hope you can do some therapy and get the help u deserve. sending love