Simulationism Anxiety Disorder
Image from Bernd Helfert
A mental disorder in which the sufferer becomes preoccupied to a degree which interferes with their daily life on whether what they experience as the most fundamental level of reality is, in fact, a bottleworld simulation running on a substrate of a more fundamental reality's computronium. Note that this disorder has nothing to do with the truth or falsity of the belief, or whether the individual is a Virtual or not, but instead is concerned about the obsessive thoughts and actions, and related anxiety, taken to a maladaptive extreme.
While the presentation of symptoms may be similar, this disorder differs from Truman Syndrome, in that T.S. is a form of paranoid schizophrenia, while B.S. is an anxiety/obsessive disorder.
Treatments One possible method of treatment, if the patient is of the right mind, is some teaching of some of the basics of rationality, critical thinking, or even Bayesjutsu training. Specifically, whatever is necessary to bring the patient to the point where they can seriously consider the question, and come up with an answer for, "In what way would you be able to tell the difference between a universe in which your belief that everything is a simulation is true, and a universe in which it is false?". If the answer that can be derived is that there is no way to tell the difference, then, if all goes well, the patient can simply treat the entire problem as one of the many unanswerable philosophical questions, and go on to live their lives without spending excessive time worrying about it. If the patient does come up with an answer, and proposes a way to tell the difference, the proposed treatment is to let them run some scientific experiments to see which answer the preponderance of the evidence points to.
If the patient's thinking is sufficiently disturbed to be unable to accept such rationality training, then their problem is somewhat deeper than the surface presentation of symptoms, and this underlying problem will need to be corrected before the above will be possible - and this sort of difficulty falls straight into the usual realm of psychology, neurology, or whatever field most directly applies to the problem at hand. Once this is dealt with, so that it's /possible/ for the patient to think straight, then, if the patient is still worried about simulationism, the approach described above should suffice.