EDIT: Because I've been getting a lot of questions regarding when the ponies' breakdowns occurred, I've decided to list them here.
Applejack - Applebuck Season
Twilight Sparkle - Swarm of the Century (pictured); arguably has small ones in Feeling Pinkie Keen, Green Isn't Your Color, and Look Before You Sleep
Rarity - Suited for Success
Rainbow Dash - Sonic Rainboom
Pinkie Pie - Party of One
Fluttershy - Best Night Ever
Whether you view what happens to each pony qualifies as a "breakdown" or "Sanity Slippage" is up to you. I based this stamp off of this motivational poster.
I hope this helped clear up some things. Oh, and I appreciate all the positive feedback, guys.
Made it just because. I mean, everyone loves psychotic ponies, right? And I threw the arrow in...just because.
My Little Pony: Friendship is Magic, Applejack, Twilight Sparkle, Rarity, Rainbow Dash, Pinkie Pie, Fluttershy (c) Hasbro, Lauren Faust
Bronies, feel free to use without credit.
Template - $zilla774