This story is a sequel to The Letter

Rainbow Dash makes it through the ceremony but she can't handle seeing Applejack with Rarity, the woman if her dreams. So she escapes to the dressing room to have a breakdown. She gets support from an unexpected source.

Note about the prequel, it's mature, but that's only because it uses profanity over a small stretch if that doesn't bother you it's still a good read. But all you need to know from the prequel that RD is in love with Rarity.