It is possible to work it out yourself. There are various tests available online to see if you have the symptoms. Finding out the the main cause is another matter. It can appear due to many kinds of reasons such as depression, anxiety, epilepsy, psychedelic drug use, vestibular migraine, drug withdrawal and the list goes on. The main reason overall appears to be long term stress and anxiety. Doctors will tell you different things, especially ones who don't specialise in mental health disorders because dp/dr is not a well understood or well defined condition in medical literature. Have a good read up of the main symptoms and think about what was happening in your life when the condition started and hopefully you can work out the causes. Then you can think about what you can do to get better. If you have been checked out by doctors and given the all clear then I would say your dp/dr is most likely caused by anxiety issues.