I'm sure I'm not the first one who has thought of this.. The real hardship of Celestia is simply that.. She has not slept while Luna was banished
Evidence:
If you look at Lesson Zero, The sun was ticking down as the day went on. This would mean that Celestia had to constantly move the sun, Which would be fine beside the fact that she took on the responsibility of raising the moon and sun since Nightmare Moon was banished. If she didn't there wound't be afternoons or dusk, If a beloved princess turned villain was forgotten in a span of a thousand years, surely a simple concept of dusk could too.
Which would also give Celestia an Alibi on why she didn't help the mane six on the first episode. She had to keep the balance by constantly using magic.
(Yes, I do know that her horn didn't glow all the time, but consider her age and knowledge living for thousands of years, She could have found a way to conceal her magic to keep her citizens from worrying about her well-being)
I'm sure there are holes in this theory, please let me know what you guys think and I'll try to improve it to the best of my abilities..