Here's what I remember...

• Twilight was more or less the main character.
• I forget exactly what happened or why, but Twilight went to another version of Equestria.
• I forget exactly what was going on in it, but the alternate universe version of Equestria was kind of a "dark" version of Equestria. One of the princesses was ruling like a tyrant, as I recall. Or maybe it was Discord. I can't remember.
• Twilight meets a group of ponies that were a sort of resistance movement.
• One of the ponies was, if I recall, a white mare, who was... I think somehow related to the princesses, maybe? And she was a fairly powerful/dangerous figure
• I think there was a romance between Twilight and the above mentioned OC.
• Twilight eventually does get back to her home dimension, after defeating the big-bad, and I think the OC followed her.

Does any of this ring a bell? I know it's a lot of vague details.

7755557
Sounds familiar

7755557
Can’t say I’ve heard of it, but now I wish I had. That sounds like a really interesting story.

Wasn't the resistance leader like Cadance while the tyrant was Celestia? I think.
I do not remember but I too would like to read it again. If it's the same story.

7755888
Same.

7755649
7755888
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Well, I found it, finally. Downloaded the FIMFictionArchive, converted the entire thing to plain text, and then spent about two days using AgentRansack to search for random collections of words that seemed to feel right.

It's not, as I remembered, a "Twilight-centric" fic, though. There's an A-plot and a B-plot, and Twilight is the A-plot, but there's still a lot of other stuff going on. It's also not a stand-alone work, it's one installment of a multi-work series. And a decided NSFW one, if that matters. It's Who Rules?: Shattered Hearts, the fifth installment of the Who Rules series.

Other than thinking it was much more Twilight-centric than it turned out to be, the rest of the description I originally posted pretty much fits.