Author's Note: I have gone through the process of rewriting my story
The main pairing IS Twilestia (don't like, don't read), there will be mentions of Flashlight, as a past relationship of Twilight's.
By the way:
'Whatever is marked like this' - is a flashback / memory
"Whatever is marked like this" - is thoughts.

Enjoy!

"Go and you'll see that a smile, often hides a great sorrow"

This night was not like any other nights, that much somepony could see - Princess Luna's nights were charming, ethereal, enough to make you stop and look at nature in it's raw essence, galaxies spreading far and far away. It was very much different than what Celestia used to create during the nights - as the sun diarch, it was pretty difficult for her to capture the very feeling night was supposed to give - it makes sense why the days were so beautiful and brilliant, that nature awoke with every kiss of sunrays on the leaves of trees, on the petals of flowers. No matter the season, daylight was time for joy, while midnight was time for fantasies.
Princess Celestia was thinking this as she looked towards the endless curtain of night, adorned with billions of stars and constellations - some old, some new - or some being Luna's own design, as it was the constellation that formed Starswirl the Bearded's figure or the one that looked a lot like Twilight Sparkle. The princess paused in her thoughts to recall each memory she shared with Twilight during her years of studies - studies the latter insisted that she continues even if she's now an Alicorn and the Princess of Friendship - and Celestia came to a bittersweet realisation, that her feelings towards the young protege have changed to a great extent than she thought. She still saw in Twilight that filly - oh so devoted - to doing everything in her power to please the princess, but those years haven't changed the fact that Twilight was now a mare. A lavender mare, with that superb six-starred cutie mark, with her big purple eyes and that beautiful mane Celestia cared to admit she brushed as the two were spending time together.
"Yes, but then you were her mentor, you were her guide. You weren't that much of a confidante, of a friend, of someone she could trust more than anyone", Celestia chastised herself. "but then again she still trusted you with taking decisions." The white Alicorn was fighting herself over the dreariest of matters: a matter of the heart. A matter where feelings of the past turn into something different in the present, and Luna was keen to observe the struggle Celestia was going through - the ever-so wise Lunar diarch knew her sister like the back of her hoof - and the banishment didn't diminish the strong bond the two sisters shared. A thousand years is a huge gap, but they could always catch up as time went by - and Luna thought it was the right moment to delve into her sister's mind...
The Lunar diarch descended towards the balcony where Celestia stood - her attire resembling the stars on the sky - and looked with her azure blue skies at the white Alicorn, concern written all over her muzzle, and it was obvious Celestia refused to look at her sister, she didn't want to trouble Luna too much. But the Lunar Princess wasn't the one to give up so easily.

- Tia, please, look at me. What happened to thee? Why thy sadness is so ever-growing? Luna asked her sister.

- It's nothing, Lulu, Celestia tried to diminish her sister's concern.

- Thy muzzle says it's nothing, but thy heart says it's something. Why are thou so adamant in bottling up whatever is disturbing thee? Luna frowned, as her eyes kept gazing at the white alicorn.

- Lulu, I insist, it's really nothing. I was just-, Celestia tried to say as she prepaired to go back inside her bedroom.

- Don't say it's nothing, Tia. I can sense your sadness, and I feel it's related in some ways to Twilight Sparkle, Luna said as she stomped one hoof on the balcony.

- What about her? She's my protege, and I must say the lack of friendship reports has made me sad, Celestia lied through her teeth, I had hoped she will at least write to me some more, that's all, the alicorn said as she dismissed the discussion, entering her bedroom.

Princess Luna was right, Celestia was bothered by something related to Twilight Sparkle - and she didn't buy through her sister's lie. "Whatever is the reason for Tia's sadness, she will come to me, sooner or later."

On the other side of Equestria, in the small town of Ponyville - where Twilight has been residing for a long time, after moving from the Oak Library to her Friendship Castle - the silence was interrupted by a few crickets here and there and a few sleepless ponies that decided to enjoy Luna's beautiful nights, as their sillhouettes were given away by the few lit lanterns in various parts of the town. What wasn't entirely surprising was that, in the distance, one could see the lights on in Twilight's Friendship Castle - specifically, one light, somewhere at the ground floors - because Twilight enjoyed studying at night sometimes, and there were days when she didn't get out of her castle, as she was sleeping during the day. Her hectic schedule was somewhat criticized by Spike and the rest of her friends - even Celestia pleaded that she'd rest - , but Twilight reassured everyone she is just fine, even though that wasn't entirely real. The young Alicorn was indeed troubled by the fact that her studies were eating up a lot of her time - not that she didn't enjoyed them - but she didn't see anypony for days, and no matter how hard Rarity or even Applejack tried to convince her to hang out some time, Twilight would always refuse.
Twilight was also plagued by various thoughts, about Celestia, about her studies, about pretty much everything - and the only thing helping was Cadence's exercise to breathe in and out, while holding your hoof away - but even that method was prone to failing sometimes. Right now, she was studying The Great History of Equestria - Second Book, mainly because she needed to get some facts straight and remember as much as she can. Twilight was going by the idea that "if you analyse everything thoroughly, you're bound to succeed", and that kind of thinking did save Equestria a number of times - for example at her brother's wedding - she never forgot the moment Celestia was hurt, her heart then was galloping so fast, fearing for the Princess' safety.

'- Princess Celestia! Twilight yelled in fear, as she trotted rapidly towards the princess, her friends following behind.

- Twilight...the Elements of Harmony...you must get them if you want to defeat the Queen!'

"Looking in her eyes, I could see determination and fear. She feared for us all - but she feared for me the most - I could see it." Twilight then recalled how she heroically stood, alongside her friends, forming a circle around the Princess - the alicorn remembered that she did try to recover on her own terms, but she was weakened.

'- I might not know the full details of how you managed to capture Cadence and everything, but I do know one thing, Twilight said as she gritted through her teeth, that nopony hurts Princess Celestia and goes away with it!

- You do realise that I'm stronger than Celestia, you foalish pony? What can your magic do? NOTHING, Chrysalis was roaring as her green eyes were shining with hunger.

- I will do just about anything to stop you, Chrysalis! Twilight's eyes were filled with anger as she charged up her horn.

- And we're goin' to stand together with her, Rainbow Dash said as she advanced next to Twilight.

- Ah know one thing, Queen, and that's nopony messes with us all, Applejack trotted forward.

- We stand as one, Rarity added as she joined Applejack and Rainbow.

- You foalish ponies! You have just signed your doom, Chrysalis said as she charged her horn with Changeling power.'

Twilight then recalled the rather fierce exchange of magic between herself and the Queen, and how at one point Chrysalis changed into Celestia, forcing the Alicorn to hold back, like a lot, until the real Celestia urged Twilight to defeat the Queen. "It wasn't the best day, but then again, Equestria was saved, and the Princess didn't suffer any long term injuries. Why do I still feel sad though?" the lavender Alicorn rummaged the thought in mind, while returning to her studies, but the frown on her muzzle did not disappear. In the meantime, Spike, awoken by the need to quench his thirst, descended from his bed and out of his bedroom - which was two rooms away from Twilight's - then he started descending the stairs leading to the ground floor, where the kitchen was.

As he finally descended the 150 stairs - while admiring the night rays that were kissing the window frames - he noticed that down the corridor, a pale light was fading from a creak, and figured as much that Twilight studied in the night again. "Maybe I should check on her, see if she's okay?"Spike thought as he approached the study and gently opened the door.

- Twi? Are you okay?...

- Sweet Celestia, Twilight yelped. You scared me, Spike, the lavender alicorn said as her eyes were filled with fear.

- Sorry for that, Twi. I wanted to make sure you're okay, the small dragon said.

- I'm fine, Spike. I'm just studying The Great History of Equestria...

- At this ungodly hour, Twi? I know you study a lot during the night, but it's still not good for you, Spike shook his head.

- You're right, Spike, I should be resting, and I will, I promise! I need just two more hours and I'll be done with this, Twilight said as she looked to the dragon with worry.

- Please, Twi, I'm saying that just for your own good-

- Yes, and I know that, but please, let me finish studying, Twilight insisted. Good night, Spike, the alicorn said as she gently pushed Spike through the door and closed it behind him.

"Why do I feel like she will sleep in the study - again - ? It's already common habit for her to just dooze off in the study after a long night's study. I know the Princess pressures her to know a lot of things, but even she would ask Twilight to rest well..." The dragon went down to finally quench his thirst with a small glass of water, and then back to his bedroom, where he cuddled in the small basket Twilight designed for him, while the Alicorn was still deep down in studies.

Many hours later...

The sun was slowly rising, as Celestia cradled him while whispering "Sun, sun, hear my plea, rise above all of Equestria", and then the sun went on following his course. The rose-like colour of the sky when the sun was rising impressed the sun diarch - after all, it was indeed a miracle of nature, this intriguing mingle of the azure blue and golden - and it was a view leaving out hopes, dreams...plans for the future, at least that was what Celestia thought it meant. However, their entire world was soon going to come crashing down, and the sun would no longer be the moral compass of Equestria...