I gotta say, Amending Fences was definitely one of the hardest lessons any of the Mane 6 have ever learned.
I think that Fluttershy's hardest lesson to learn..or at least one of them..was "It Isn't Easy Being Breezy." I say this because Fluttershy is a compassionate pony whom cares soo deeply for the animals around her. So when the Breezy's have to stay at Fluttershy's house, she is more than happy to oblige...much to the chagrin of their leader. A few days go by, and she is told that she must part with them..even though she constantly cannot seem to part with them. Finally, she has to let them go to ensure that they are able to get home. I think that is one of the hardest lessons she had to learn...which involved having to show tough love to the Breezies and help them get home.
That is what I remember of the episode, so pardon me if this was a little odd in the description.