Have you ever gotten so drunk you forgot you offered the princesses' hoof in marriage to whoever brought your sister back? And how many 'es' there are in 'Princess'?
Rarity always wanted a dream wedding. Now that she's about to marry the dream girl, she's determined to make the rest just as perfect. Of course she'll overthink this.
It's the hours after the disaster that was the Gala. Rarity tries to pretend her feelings weren't crushed by all of it and fails miserably. This gets noticed.