^^ I have a few mental disorders. I think the core of being realistic is to understand, that the world of a person with a mental disorder is perfectly ok. If you were alone on this world... everything would be alright. It's the clash with other people that brings the problems.
The first step is to pick an instability you want to portray. A phobia? Autism spectrum disorder? Schiziphrenia? Paranoia? (Please don't say depression. Depression is... where's the fun in having a character without ANY motivation?), something more spectacular? DID? Some sociopathy or psychopathy?
When you picked one, go and research it. There's plenty of sites that provide information about mental disorders. You can find a relatively nice summary about mental disorders
here. Research it in depth. And then try to imagine, how suffering from the conditions of the disorder would affect you in your everyday life.
Again: If i didn't have to interact with people, I'd be fine! It's interacting with the world around you that causes the thing to be a "disorder". MY world is just fine. ^^ As I said - I have a rather funny collection of disorders, but ever since I live with these since my birth, I have come to terms with most of them. I am a highly functional person in the real world. Some of my disorders come in pretty handy at times (You've got to love seeing patterns everywhere when you are working with lists!). A "normal" person with a mental disorder is going to do SHIT to show it to a stranger. You function as good as you can, day after day after day. And then there's the occasional "creepy" moment for others. Like... when you are HEAVILY overtriggered, everything is getting way too fucking much and you suddenly DO yell at the voice inside your head to shut the fuck up - while you are standing at the bus stop. And of the three people waiting there with you nobody said a single word.