4 years later:
Luna was standing on a balcony looking at her moon as sunrise time approached and she started looking back at how she has progressed during these 4 years since the Tantabus.
It took her a year before she started going herself at the astronomy association and she couldn't have been more happy at how well she was received, the ponies there were very enthusiastic to learn about the night sky from Luna and she was eager to talk about it to them.
The field itself has improved as the help of Luna has given the ponies direction on where to improve and increased the number of ponies, especially the young ones that have found fascination in astronomy.
It still took her another year before she gathered the courage to encourage a field that she wasn't an expert herself, like art where while she's interested in it and she draws for time to time she didn't give it the interest she gave to astronomy. Still she eventually started to donate funding to art schools and eventually when the ponies started to let go of their fear of her, she invited them to the palace to showcase their work.
The fear from the ponies took around 2 years before it started to noticeably disappear as her previous work started to change their fear to wariness that while not ideal at least it gave her the chance to talk without them running away.
It was also around this time that she finally managed to catch up to all changes made since her banishment, now she could focus on where and how to improve the nation, but that is likely to take a long time and even longer for any changes to take effect and as immortal she had all the time in the world to wait for them.
As the 3rd year from the Tantabus came she decided to also show herself in the dreamworld to the older ponies which she didn't do until then because the few time she had tried until then had ended with the ponies either going back to a nightmare at the sight of her or screaming at her to leave their dreams as they thought she was the cause of their nightmares.
That happened sometimes but it was mostly seen as helping considering she only showed herself to the ponies with the worsts nightmares or recurring ones, for Luna normal nightmares the type that happen once occasionally where something that ponies could resolve themselves and her helping with those could cause them to depend too much on her to solve their problems.
Now 4 years later she found herself in a position she only dreamed of before. Most ponies accept her (not loved but, acceptance was something she rarely got even before her banishment so Luna was happy about it), she found some ponies that loved her work, she could her duties without having to fear her subjects running away or demanding to banish her again (this was mostly the nobles but eventually after they realized it couldn't do it, they bredungly accept her).
She still had long periods where she was only by herself but it was something she's come to accept as part of being the night princess and as long as she wasn't alone all the time she could deal with it.
As for Luna's relationship with Celestia? It's mostly pretty well, they are mostly like usual siblings again without the worry of Luna reverting back to how she was before the Tantabus incident and the reunions helped resolve almost all of their problems between each other, "almost" considering Luna thought that Celestia still didn't fully realize how exhausting her night duties were now she had some more people in court along with helping more on the dream realm and the occasional visit to the astronomers as Celestia cheerfulness at waking up were too much for Luna to handle and has give way to the occasional spat.
"Luckily our spats on that matter haven't gone too far yet, but I can't stop thinking it will really come to a head soon," Luna thought.
As if summoned Celestia appeared at that moment, stopped at her side and spoke. "Hello sister! How are you? Ready to start the day?"
"Sure, let's get this done," Luna responded.
"She still hasn't said anything about the flowers I put every night, does she really not care about them?" Luna thought.
Changing her focus she concentrated on lowering the moon, sure with her experience she could do without it but if she concentrated she could make it seem more pretty and if combined with the same concentration by Celestia on her sun it gave some espetacular starts to day and night.
As they finished and the sun was at its correct place, Celestia spoke. "It has been a long road, hasn't it?"
"Yes, and where do we go from here?" Luna said.
"Can't say, Luna but I know that the future is bright for us," Celestia smiled.
For Luna the way Celestia accentuated 'know' gave her a clue that she did know something about the future. "What could it be, another prophecy or vision? Wonder why she wouldn't say it to me, still I'll trust her she'll tell me in time, she hasn't been wrong with Twilight yet so I hope she won't be wrong with this."
"I hope you're right, well in either case, at least we'll deal with it together right?" Luna decided to say.
Celestia hugged Luna with her wing and said, "Always sister, always."
Luna nuzzled Celestia and they stayed like that watching from the balcony until they had to leave. As her mind went back to how impossible this was for her before she thought, "Guess that sometimes dreams do become a reality."
End.