It’s not love. But it’s easy to pretend it is, when she’s so cute and great and so reminiscent of her. Looks-wise, they aren’t that alike. Eyes of gold instead of blue. Blonde hair that goes down instead of up. Grey instead of pink and of course, wings. But you just had to look deeper, and Twilight was very good at that.
The eyes had the same… not innocence, no, perseverance, perseverance in believing that despite all the awful things they had seen, ponies were naturally good, and life was great. The hair was just as innocently disorganized, the mare too busy with some endless wacky adventure to even realize that she hadn’t tidied it up. But that was all secondary to the pony within.
Derpy was so good. Bouncy and caring, always trying to cheer up ponies no matter how they treated her before. Whether they were laughing with her or at her didn’t matter. She was clumsy, could hardly stay in one place. She had no self control, experiencing the greatest happiness and the lowest sadness. She was amazing. So you may ask why, when Twilight spoke of her so highly, did she not love her? The answer’s simple.
Twilight loved Pinkie. And she loved only the Pinkie part of Derpy. They had differences but none substantial, and whenever one surfaced Twilight would just remove it from her memory, and subtly make sure Derpy stayed more in line with her Pinkieness. It was wrong and cruel. It was not love, and Twilight knew Derpy knew this. Derpy was not dumb as so many ponies assumed. She had the same unique intellect as Pinkie. But Derpy still loved Twilight so much, so completely and devotedly, the way that Twilight had always imagined Pinkie would be with her.
The way Pinkie was with Rarity right now. Rarity, the unicorn who was everything she wasn’t. Smooth, romantic, classy, a looker. She wasn’t an awkward bookworm, a dangerously overeager student, a condescending, annoying, boring little egghead. She was Rarity, beautiful fashionista, famous dressmaker, the catch that anypony would never let go of. Twilight could never compete with that, and if she was a good pony, she would’ve accepted that, tried to move on, and if she couldn’t, would’ve had the decency to suffer in silence, alone.
But Twilight was not a good pony. And when in every social occasion they would share pecks or nuzzles or just perfect looks of true love, Twilight could not bear the pain naturally. She needed anesthetic. And Derpy was that anesthetic. When she passionately kissed that mare who would kiss just as passionately back, when she heard that mare’s special giggle only she would hear, she could pretend that Pinkie was with her, and Rarity was the one pretending, the one stringing along a good mare like Derpy. But Rarity was a better mare than Twilight. That’s why she got Pinkie. That’s why she deserved Pinkie. A good pony would know that, and overcome these awful feelings. And if this was a romance novel, Twilight would come to love Derpy for being Derpy, would be happy with her and happy for Rarity and Pinkie. If so many things were or were not, this could be love. But Twilight was a very bad pony. And this is life.
And so this was not love.
That was rough. It was interesting to see Twilight be so honest about what she was doing while also deluding herself about it at the same time. It was tough to read about Twilight erasing the things about Derpy that weren’t like Pinkie, and just how far she was going with her facade was really heart-wrenching. But I think that the last paragraph before your final sentence there was the most powerful: Twilight knew that things weren’t good, and even farther than that she knew that she’d never come to love Derpy for herself. And that’s the most tragic thing of all. Well done!