Been rereading RH Junior's Parting Words when I had a sudden epiphany: All that stress of being Celestia's student, all that studying, all those tests from both the school and her mentor, Twilight must have had quite a stressful foalhood. All that stress at such a young age, it's no wonder Twilight's so neurotic and and anal-retentive, and a gods-damned miracle that she's not suffering from a more serious medical condition.

But what if she did?

What if, before the start of the series proper, while Twilight's still a filly, all that stress actually causes the likes of a psychotic break, or even a full-blown stroke or heart-attack (just a few examples, feel free to use what you want.)?

The idea wouldn't be about whether or not she recovers (she does, at least to a point), but how much things would change if such a thing were to happen. How would Celestia react? How would Twilight's family react? How would Canterlot, and possibly all of Equestria react? How much would Twilight change? How much would Twilight's relationship with Celestia change? How much would the future change?

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How would Celestia react?

Ditches her and goes with plan B to deal with Nightmare Moon. She'd make the ditching as gentle as possible to avoid overly hard feelings, but she can't afford to waste too much effort on a broken tool.

How would Twilight's family react?

They'd be horrified and heartbroken that such a tragedy and seek answers to find out how it happened.

How would Canterlot, and possibly all of Equestria react?

Twilight who?

How much would Twilight change? How much would Twilight's relationship with Celestia change?

It depends on whether or not the shock of the tragedy in question shakes Twilight's confidence in Celestia. Twilight might consider her weakness proof positive that she's not worthy of a relationship with Celestia. She might seek some sort of retribution against her mentor for putting her under such stress. She might even grow up a little from the experience, recognize that nobody can be perfect, and move on with her life, sadder but wiser.

How much would the future change?

That depends on how good of a Plan B Celestia. Assuming the country doesn't fall apart, it depends on how bad a shape Twilight is mentally and psychologically. Twilight would still have the potential to be a very skilled, powerful, and knowledgeable unicorn. If the experience doesn't crush her, she might well go on to have a perfectly fine life, if one where she has to avoid stress more than most. There's no reason she couldn't be a writer or experimenter or whatever else kind of jobs exist in her field.

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Ditches her and goes with plan B to deal with Nightmare Moon. She'd make the ditching as gentle as possible to avoid overly hard feelings, but she can't afford to waste too much effort on a broken tool.

Well, that was cold.

Just had a thought: You know how Pinkie Pie threw a surprise party for Twilight on her first day in Ponyville? Imagine that if Twilight had a heart condition due to a heart-attack during foal-hood. I can see Twilight trying to keep herself calm, less she has another heart attack, and Spike coming to Twilight's defense and scolding Pinkie.

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Cold as the ninth circle of hell. But the ability to be cold and calculating when necessary is vital to being an effective leader. Keeping deadweight in important positions for sentimental reasons is tyrannical.

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True. In the end, no matter how much Celestia may care for Twilight, Equestria must come first. Such is the price of the crown.

Although I was mostly describing how you phrased it.

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It wouldn't be the first time she did that.