[THIS STORY IS ON INDEFINITE HIATUS. IT MOST LIKELY WILL NOT BE COMPLETED.]
It began after the coronation. These random things. Rainbow Dash suddenly feels that she can't live up to her own standards, and that she is a complete failure. Twilight Sparkle can no longer control her temper.
These random, seemingly insignificant things.
But when the two clash in a tragic event, both ponies realize that it is not at all normal, and cannot remain that way.
For the sake of all good things... they must fix whatever it is that has changed them.
(Based off of "Star Trek TNG" series finale, "All Good Things...")