Introduction: In Which Celestia Makes Luna Regret Her Nightmare Moon Era For All the Wrong Reasons
It was a beautiful sunny day. This concerned Luna, as it usually meant her sister was buttering her up for something. Like... Oh nooooo. Community service day.
You see, dear reader, Celestia saw fit to punish Luna for trying to plunge the world into eternal night. Metting out punishment for such an offense is certainly reasonable and to be expected. But community service? That's just cruel and unusual.
Now, I know community service at first glance sounds readily agreeable. Scrubbing graffiti off walls, cleaning the park fountain, giving alcohol awareness training to Berry Punch, patching up rifts in the space-time continuum caused by beings beyond our ken and understanding, etc. And you'd be completely correct.
But Celestia adhered to a dusty and archaic philosophy that the punishment must fit the crime. One can't help but wonder [Okay, seriously readers, please help me figure out something witty to put here! 
], but that's another story for another day. And since Luna tried very hard to ruin a lot of pony's lives, it seemed only fair that through the karmic mechanism of community service, that the reverse be true. Today, Luna would be providing psychotherapy.
Oh, most patients weren't too bad, it's true. Dinky Hooves was hyperactive. Comet Burst had endowment issues. Applejack Apple thought she was actually an apple fruit. Stuff like that. But then she had patients like the megalomaniacal Trixie Lulamoon or the conspiratorial Bon Bon Sweetie Drops, patients who made her question whether rebanishment wasn't such a bad idea. And then she had her sister.
Wait, what? Luna looked closely at the schedule her receptionist had hoofed to her a few minutes before. This was a thorough inspection, not just a casual glance. And lo and behold...
"Hellooooooo, Luna!" Celestia declared as she entered the makeshift doctor's office. It was a spartan affair, carved out of one of the conference rooms tucked within the town hall. But it got the job done. The office that is. Luna unfortunately still had 136 hours of community service left. So her job was far from done. Which Celestia had just painfully reminded her of.
Luna's eyes grew wide. "No. Oh, hay no. I am NOT giving you counseling!"
Celestia adopted a fake pout as she stood there in the door way, ducking slightly so her horn would not scrape the ceiling. "You mean you don't want to spend time with your sister? Luna, I'm truly hurt." She harrumphed and raised her muzzle high in disgust.
Luna rolled her eyes and sighed. "Tia, don't do this. I love spending time with you. I'm so incredibly glad to be back." Her eyes watered a little, as the lunar mare reflected on how much less sucky her world was these days. Except for one thing. Her eyes ceased watering and instead hardened. "So why ruin that by making the activity we partake in together one I hate so much?"
Celestia shifted uneasily. "Geez, sister, take a chill pill. It's me we're talking about. Not some crazy narcoleptic pony or schizophrenic bookworm."
Luna grumbled before responding: "One: Yeah, no, your brain is hella scary. Two: Twilight has OCPD, not schizophrenia. Three, oops, so much for patient-doctor confidentiality agreement. Four: Oh well I'm pretty sure you already knew that though. But seriously, the two conditions, while occasionally comorbid, express themse--"
"Luna," Celestia interrupted, "I wasn't talking about Twilight. Seriously, relax. And really, am I that bad?" She raised an eyebrow incredulously.
"Okay, seriously stop standing there in the doorway; you're freaking me out. Just sit on the damn couch already. And yes, you have forty-three thousand years of mental detritus stored up in your noggin behind that aggressively calm demeanor you front. I am absolutely terrified of your mind and the contents within. However, I love you enough that I'll suffer through it for..." Luna flipped through her notebook with a blue glow... "Oh geez, an hour session? I'm doomed."
Celestia chuckled as she trotted over to the couch, which creaked under her weight as the matriarch struggled to find a comfortable position. "That's the positive spirit. Now, Doc, are you ready to help guide me through my issues?"
Luna scrunched her eyes as her ears pinned back. "Yes," she whispered in apparent pain. "Yes. Ms. Solaris, how can I help you this morning? This lovely morning I was meant to spend outside but am instead doomed to languish far away from, for in my hubris I plunged the land in at-the-time-seemingly eternal night. Alas, what a fool was I. For I became proud and haughty, and rather than use my goddess-given mouth to communicate with you, instead withdrew from both the public eye and my relation with you, becoming ever more bitter, an icy, hardened mare who only wanted recognition, only wanted love, only wanted the populace to see and acknowledge the beauty of my starry skies, the creation of which I put so much effort into. But alas, I was not praised, and in my self-imposed rejection, I rose up and declared myself mistress of the nocturnal age, a new leader under whom all would rightfully worship my surpreme glor..."
Luna blinked suddenly. Celestia followed suit. A blush appeared on the former's cheeks. "Sorry. I got away from myself." She cleared her throat. "How can I help you?"
Celestia grinned amiably. "Stirling soliloquy, sis. As for my issues? Well, have you ever wanted something, but aren't sure if obtaining it is a good idea?"
Luna's body visibly relaxed. Thank Yatzee, something benign "Of course. Desire is a complex emotion that many ponies must work out, myself included. What is it exactly that you want?"
"I want to fuck Shining Armor."
Cadance take the reigns, this is going to be a loooooong session