Celestia was old. Sometimes the fact shocked her, thinking about it. She didn't feel ancient. She still loved and cared for her people, friends and sister. She still cherished art and food and theatre. She wasn't weary or sickly or senile like how some of her loved ones had become over the years.
But the fact remained that she was older than any other being in her people's kingdom, even if only by a few minutes in one case (Luna was still irritated over that, no matter how much Celestia tried to tell her she was just as much her equal, that she didn't care about the few paltry moments that she had been devoid of her other half).
She had seen wise advisors grow slow and mighty soldiers weak. She had seen illustrious legacies forever tainted as their heirs grew cruel and conceited. She had watched as the horrors and nightmares of Discord that she had fought so hard to stop, that she still woke up in terror at the memory of, that still caused her heart to break sometimes when she looked at Luna- he had conjured creatures that looked like foals to fight them, ravenous monsters that ate pony flesh and drank their despair, that looked so small and beautiful and like Luna when she had been young. Even now, her subjects still spoke of demon children birthed by the Wicked Moon. Even now, she still remembered how they looked, how innocent they appeared, how her eyes burned with tears as she- . . . even in his imprisonment, Discord was a gift that kept on giving.
Yes, Celestia was old. And so, she was far more able than her subjects to control her shock at some of the things she had learnt on this trip. But that didn't stop her worrying. Because how could she not? She could feel the disharmony radiating from this land, not from the Empire itself, but an echo, a dark whisper, a lingering taint that muttered of misery and shadow and fear and war, of cold hate and pride and entitled greed and a scream, a scream of a thousand dying soldiers, of slaves and, finally, death. That last tune, so quiet compared to the rest (which was itself faded), didn't leave her angry or fearful. It just left her sad. Was this her people's future? Hadn't she and Luna sacrificed enough? Hadn't all those who flocked to them, who fought for them, who loved them done enough?
"But we won't be caught off guard," Celestia thought. Her ponies wouldn't be undefended from slavers or pirates or darker things. This land was cold and fierce and harsh, and its inhabitants were so different from her own, but they still loved and cared for one another. She could feel the Harmony from them, a hard one, but one that promised protection to those that joined it. It wasn't ideal. It was alien and strange to her, and some of it was close to anathema, but she recognised parts that Equestria needed if it were to survive and prosper in this new world.
But she still couldn't help but feel slightly hollow, thinking of legions of unicorns, earth ponies and pegasi marching in formation, of her and Luna donning armour again, of a hundred thousand graves of endless potential. She remembered the moment when she stopped and found herself on top of a mountain of dead foals.
"What's wrong with peace?" she said to herself wistfully.
AN: I never thought I was going to write Princess Celestia angst, but here I am. I really wanted to explore Celestia as the Big Good she is, who isn't perfect but is still benevolent and loves deeply. She's old, she's lost so much and she's still traumatized from Discord's favourite game of "let's throw Nightmare Fuel at people!", but even now's she's a canny operator and determined to do her best.
It probably goes against canon somewhat, but I sort of envisaged the Princesses amassing an army of supporters that waged a weird sort of insurgency before they were finally able to defeat Discord. It wouldn't be enough to combat the dude himself, but just enough to kill enough creatures that Celestia and Luna could reach him for a duel.