So with Exit, you need to have been diagnosed by a psychiatrist. So it probably takes some time, and that time has to be obviously paid. After that, a second psychiatrist checks the diagnosis for any errors. And the third actor is going to be a regular doctor, double checking for errors. If all goes well, you get the death cocktail. As things can go wrong with that, you should be accompanied by someone experienced. Exit provides this assistance free of charge, as they use donations and membership fees to pay for that.
I cannot tell you about what diseases are successful. Usually, psychological issues are not enough to get the diagnosis you need. You need to be heavily impacted by it and there has to be no cure.