I read “Letters he will never read” and it gutted me.  So, here’s a cope ending for myself.
[Time dilation plays a kinda important part, (You) can figure it out]

>You are Anon
>And a decade of research, subterfugue, bribery, and blood has finally come to fruition.
>You hold in your hands a leather-bound tome, the cover cracked and dulled with age, the pages yellowing and the ink fading.
>It contained the last scraps of archaic formulae for the rift-tearing ritual.
>The journey here had cost you an arm and a leg.
>Quite literally - your left arm and leg are void of their biologic nature, replaced by cold steel and warm servos.
>It was worth the price.
>It was the way back to Equestria.
>Back to your friends.
>The place where you truly belong.
>
>Twilight had told you that the portal would be two-way, that you could come and go as you please, that everypony you had come to care for would visit.
>The last thing you saw of Equestria was her waving, before the rift between your worlds closed.
>It did not re-open.
>
>You waited.
>Seconds turned to minutes.
>Minutes to hours.
>Hours to days.
>You lost hope a month in.
>
>You were deep into thaumaturgic research within the year.
>Doing whatever was necessary to study the weird, the wild, the esoteric.
>You just wanted to see your friends again.
>It was all so…
>Bleak. Tiresome. Depressing.
>The happy memories kept you going for a while.
>While they have not faded, they stopped driving you after four years.
>
>The next six were powered purely by spite.
>At God, Allah, Odin, the Flying Spaghetti Monster - whoever ran the show behind the curtains.
>You would break the chains of this world if it was the last thing you did.
>
>And now, here you stand.
>Within a room lit only by candlelight, thaumaturgic formulae spread before you in rings of chalk and your own dried blood.
>Within your hand of cold steel lies your collection of notes, research, runes, and other assorted items.
>All so you can leave this world behind.
>Beside you is a well-worn medium-sized backpack, full of what few belongings you chose to keep over the years.
>Mostly extra research, or things you thought ponies would find nice.
>You breathe in a shaky breath.
>It’s time.
>
>You are Twilight Sparkle
>And your daily routine has remained unchanged for the past year.
>Wake up, dried tears over the bags under your eyes.
>Re-read the portal spell for the thousandth time.
>Try it again.
>Spike brings you breakfast.
>You ignore it until you almost collapse from hunger.
>You eat what little you can bear.
>You re-read the spell again. Quintouple check everything
>You try it again.
>And again.
>And. Again.
>Spike brings you lunch.
>You eat what little you can bear.
>You. Keep. Trying.
>You’ve hardly seen your friends since he left…
>It’s your fault.
>YOU did something wrong.
>It’syourfaultit’syourfaultit’syourfaultyouFAILURE
>You collapse, sobs racking your body.
>You have to keep trying. There HAS to be a way to fix this…
>…Maybe another letter would help.
>
>This is the 375th letter to Anon you’ve wrote now.
>You know he will never read them, but…
>It still helps.
>Sometimes.
>Your face remains soaked in tears as you sigh, stowing your quill and ink once more.
>With slow hoofsteps, you begin your pilgrimmage.
>Back through the library.
>Up the stairs.
>Two rooms down from yours.
>Anon’s room.
>At least…it was.
>You’ve done this enough times to wear a shallow groove in the floor.
>You gently grasp the handle in your magic, opening the door the same as always.
>Expecting to walk over to his desk, and place the letter on the ever-growing pile.
>Letters from yourself.
>From your friends.
>Neighbors.
>A Princess or three.
>Except…
>That is not what happens.
>Instead of the desk and letters burned into your frontal lobe, a tall, familiar shape stands before the desk, shuffling through the letters.
>A worn bag at his side.