What does a given pony see when they look into the Mirror of Erised?

The events leading up to the discovery of the mirror and the heart's desire of each pony that looks into it.

The continuation of "Heart's Desire" due to character limit. It is highly suggested that you read at least chapters 1 and 3 of "Heart's Desire" before skipping to this story if you plan on skipping. Lastly, the first chapter is not a pony, but a recap of the first story for those who do not plan on going back to "Heart's Desire" before moving on. *Warning: it contains spoilers.*