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 
`anmari has been spreading her infectious positivity throughout our community for over 6 years. Throughout this time Ana has been at the core of all things devious, passionately developing an eclectic gallery, helping organise devmeets, participating in chat events and also recently completed dedicating her time as a Community Volunteer. We are absolutely delighted to bestow the Deviousness Award for May 2013 to `anmari, congratulations! Read More
Sta.sh
...Also, a good number of commenters apparently don't know how old this stamp really is, based on the "The Twilight part should be this:
Ironically, Twilight seems to have had the most breakdowns in the show...
Twilihgt- snapped like a twig
Rarity- mental breakdown
Rainbowew dash- nervous breakdown
pinkie pie-lost her mind
fluttershy- hissy git
amg she was so crazy -_-
esson Zero