I want to write a somewhat different take on the whole Lunar Republic thing that hits a middle-ground between Celestia being totally blameless for the Nightmare Moon thing and Celestia being an expy of the primary antagonist of Jagged Alliance 2. (Now there's a crossover idea...) The summary should run something like this:
"A thousand years ago, Princess Celestia was not a particularly wise or benevolent ruler. She was thin-skinned, quick tempered, notoriously unscrupulous in her dealings with other nations and exceedingly unforgiving in her treatment of her subjects. In other words, a fairly typical monarch for that time in history.
"Princess Luna was none of those things. She was forward-looking, liberal-minded, scrupulously honest and slow to anger. The general consensus amongst her subjects ws that she'd be a very wise and benevolent ruler indeed, if it wasn't for one small problem.
"Princess Luna was quite, quite insane."

My objective here is to take a very different look at how Nightmare Moon came to exist and what exactly it was. And in order to do that, I'd like to get the portrayal of her mental illness as accurate and sensitive as possible. Does anyone know of any good books, blogs etc that were written from the perspective of someone with schizophrenia, dissociative identity disorder or other mental illnesses? Or, if it's not a difficult subject, does anyone here have a friend or relative who's been through the same thing or even been through it themselves?

Multiple-personality-disorder. She created NightmareMoon in her mind and then she took life inside her. She now has split personalities.

Definitely something I would read any way you do it though. Good luck!!

Also no, I don't know any books or blogs like that... The movie Shutter Island is a good movie about Schizophrenia...

Hmm, I'm not sure how much help I can be on this topic, or how much help the thing I've just thought of will be, buuuuuut purely from a look at the title and a cursory examination of what you want, this fic by Aegis Shield pops into my head: What Princesses Need.
It may not be what you're looking for, but either way I would suggest giving it a look!

869811
I would probably have wanted to look at the characters, Luna and Celestia and how they were when they were young. I would not have gone with Celestia being a tyrant, instead I would have written her as very inexperienced and uncertain to on the throne. She would TRY to be the Celestia we got today but not succeeding, making stupid mistakes and dumb decisions. Plot happens and a war break out that none of them wants, they get caught up in the events, while both of them got the power to stop the war none of them does so out of stubbornness, believing that the other one is the one who should yield.

Luna eventually breaks under the emotional and physical trams of war and adapt the "Nightmare Moon" persona to hide herself behind. In the end, the critical moment before Celestia banishes her Celestia asks Luna if she would be willing to surrender. Deep inside Luna wants to surrender but is unable to discard the Nightmare Moon persona for fear that all suppressed emotions would surface so she refuse. Rest is history.

869830

Multiple-personality-disorder. She created NightmareMoon in her mind and then she took life inside her. She now has split personalities.

Dunno, it could work but then you would need to be a damn good writing to pull it off, otherwise it would come across as stupid and probably result in a bad story. I would recommend you to be very careful if you go with this idea.

870160 I suppose, but it was just a suggestion. As I said I'd probably read it no matter how it turns out.

870195

I consider the characters the most important thing in a story, the plot being the 2th and the world being the 3rd. All fimfic personality-disorder stories I have read so far have been bad because they use it a plot device instead of expanding upon it, building the characters with it.

If you want a canon character to suffer from Multiple-personality-disorder you better have damn good reason for it. You need to tell us how both this personalities are, establish them as real characters, show us how they interact themselves and how others react to them and at the same time weave it into a story where these characters matters. All this without making it awkward.

Take Gollum and Smeagul from LotR for example. Both of them are well portrayed, they have different goals and ways of talking. One of them is a murderer, obsessed with the ring. The other one is the underdog, having lived "behind" Gollum until he had a reason to fight him. This is split personalities done right.

870260 Yeah, I understand that, but my thing was that he wanted Luna to be insane and that having something to do with Nightmare Moon so I thought; Hey, this might work!

I know it would be hard, but I've seen this sort of thing pulled off before. Not in pony fics, but yeah...