You might know yourself better than a psychologist but you don't know psychology better than a psychologist
This is an argument I see with fakers and it drives me insane. ''I know myself better than a psychologist'' but do you know psychology better than a psychologist? Reading symptoms of a list isn't the same as actually understanding and being able to recognize what a disorder actually looks like. Most people recognize themselves in the list of symptoms for a whole lot of disorders, not because they have them, but because a lot of symptoms can also be normal behavior, it is often more like poison. Poison is about dose, water can be fatal in a too large dose. Liking when things are neat and orderly isn't OCD, and it isn't necessarily disordered, to be disordered it has to cause significant damage.
And also.. we as humans are actually not particularly good at knowing ourselves. Especially when it comes to mental conditions due to the very nature of msot mental conditions. Because the whole deal with mental conditions is that they often skew our perspectives in specific ways. A narcissist doesn't experience themselves as taking credit for other people's work, or having a too large ego etc, because the very nature of narcissism is that it skews this perception.