I'm actually someone whose been diagnosed (mostly incorrectly) with a wide variety of mental disorders throughout my life, so this topic actually ended up hitting a little too close to home for me, particularly because of my knowledge on psychology and personal experience with it. I pretty much had doctors diagnose me with one thing after another throughout the vast majority of my childhood and adolescence, mostly just because my peers and family and schools didn't understand me, and the medications they put me on ended up severely screwing me up, as did the social stigma I got.
Honestly, the first 25+ years of my life for all intents and purposes got stolen from me by the mental health industry, or specifically its labelling and incapability to see outside their box, and even now, almost 8 years later, I still have to recover from and deal with the damage they did to me. I'm quite confident most people would've gone completely insane, especially since I endured it almost entirely on my own, but I managed to survive mostly due to sheer force of willpower and thanks to a deep understanding of the human mind and its psychology. I've been told that children with unusual mind states are treated a lot more fairly by the medical industry now than they were when I was young, but let me assure you, being diagnosed with something like this in my time could be a recipe to have your life ruined. I'm a walking example.
Regardless, I'm going into rant territory, so I'll post up a response here.
I also try to avoid labelling when working on a character with mental disorders, partly because I have vastly more respect for psychology than I do for psychiatry, but I would say some of the characters in my project would definitely be, from a reality perspective, classified as sociopaths, psychopaths, or narcissists. Some of the other disorders mentioned by above posts apply to certain characters too. I don't openly think about these disorders when I write the characters though; I create the character first and then observe them and determine if they would get diagnosed in reality.
In a very real sense, my story is also based on my own life and experiences as much as it is my philosophy, so the main character is essentially my avatar, and he takes all of the bad in me as well as all of the good, and since I myself have been diagnosed with Asperger's (the only diagnosis of me that, to this day, is still considered to stand as true, since the rest of them were eventually dropped and I am no longer on any medications due to me pretty much refusing them from now on, which is probably one of the things that saved my life), it can appropriately be said that when I write my main character, I'm writing someone with personal experience of the disorder. Whether or not I agree I have it is up for argument, since I don't like labels, but I do acknowledge that a lot of it fits me. Depression is another one that I've had people claiming I have, but I don't think my circumstances justify calling what I have clinical depression. I also have an extremely unhealthy amount of stress, but that's probably not tied to a particularly mental disorder.
When medical professionals attempt to understand a mental disorder, they are always looking at it from outside the window. They are not experiencing what that person goes through, nor do they see the world from that person's perspective. Though science is supposed to be an "objective" study, for all intents and purposes, psychiatry is subjective; the psychiatrist only sees their interpretation of the subject, no matter how objective they claim to be. This is why people who don't have a firm understanding of human psychology will struggle to write a mental disorder they personally have no experience with, and even then they might struggle if they have the knowledge because knowing does not equal understanding. I would suggest asking more people who actually have experience with such disorders, if you can find people who are willing to talk about it, to help gain a greater understanding of their experiences. Learning about psychology helps as well, but only to an extent.
I also have a very strong ability to empathize with other people, though people might not realize it from how I act (like a previous poster said, I also experience emotions on a stronger level than most people but I am not that good at expressing them in a way that others will understand; same with most things I want to express depending on the person), and I'm somewhat infamous with people who know me, both in person and online, for being able to get into people's heads or read them; things just don't get passed me, so I would honestly say that being able to understand people on that level is far more important to writing such disorders successfully then merely understanding them on a professional or educational level, hence why I suggested getting insight by asking others if you struggle to understand them personally.
I've actually provided more help to some people I know who suffer with mental disorders than any paid professional has ever been able to give them, or give me for that matter; in the 32 years, almost 33 years, I've been alive, a professional has never been able to help me with my troubles and has usually either left me feeling empty with their superficial "understanding" of my situation, or they managed to make things much worse. The extremely rare times someone said anything remotely helpful, it was because of they had a decent empathy and showed they genuinely cared, or they could relate to my problem personally, and these qualities are not something you learn by becoming a professional; any person can do these things. Doing things by the book, per se, very often fails, especially when it comes to the human mind. My whole life, and what the medical industry has done to it, are just another example of that. Similarly, professionals often fail to help people who struggle when they try to be counsellors, because their education does not actually properly qualify them to support others; all they have is paperwork. Just having a person who can relate to how you feel, and who actually cares, means more and can help more than anyone who supposedly helps people as a career but lacks these things can ever achieve.
A personal understanding into the more complex nuances of the human mind is vastly more important and offers much more than any level of professional or educational study into mental disorders could ever give you, whether it be helping others, reading people in everyday life, or writing a mental disorder. This is also why people with certain psychological conditions also often make the best psychologists, counsellors, and criminal investigators; they are the most qualified because they have a personal understanding of the matter, not just an educational one. To be completely honest, people who lack a personal understanding of the human mind (and the complex difficulties people can go through) through insight and personal experience, are unable to truly grasp those things. This is why people who do not suffer from deep emotional wounds or psychological burdens are unequipped to help those who do, even if they try to educate themselves, because they fail to relate to and understand that world entirely. The only way they can ever come close to understanding without that experience is if they open their minds to the perspective those people live in.
If you're going to read up about a mental disorder, or look into a piece of writing from the perspective of someone with a mental disorder, I strongly recommend you prioritize cases where the authors themselves have experience with the condition, since they will be able to give you far more reliable insight than someone looking at them from outside of that world, which is what any professional is doing unless they have personal experience or have truly come to understand someone who has that experience. So long as the person is honest about how their condition effects their life, at any rate, you will learn more than any book on the subject from another source will teach you. Unfortunately, some of the mental disorders that have been mentioned in this topic, particularly those I noted earlier with the exception of Asperger's (so, basically, Sociopaths, Psychopaths, and Narcissists), are people who are more often than not known for being consummate liars, so finding reliable testimony from them could be difficult.
Edit:
@BrokenPromiseI had planned on responding to your post, but never ended up doing it since I wanted to avoid putting quotes from several people since it would make my post longer than it already is, so I was going to wait til someone else posted, but it's taking too long and I don't want to double post.
It's interesting when you mentioned avoiding making characters who don't have a reason for what they're doing since it would make them unlikeable, since sometimes being unlikeable is the point of a character, but I agree with what you're saying since I usually go into great lengths explaining the mentality for why my antagonists, even some of my worst ones, behave the way they do. As for not posting inner monologues of a character if you want to avoid them losing the audience, I do this too, including the hints and having other characters try getting into the subject's head, but once a character has been revealed to not be someone the audience should be rooting for, that's when you'll start seeing their true colours in the internal as well as external.
