Read this: Where the smile doesn't reach by Infinity Shade · 11:52pm Nov 4th, 2015
The story is here:
Where the Smile Doesn't Reach
The description is this:
Marble has always been told to just put on a pretty smile. But sometimes the prettiest smiles hide oceans of deep secrets.
When she meets a new pony, it's like all her worries and demons melt away.