I'm pretty sure it's because she thought that once he returned to the real world after the first time he'd start working to get her out of there, but didn't do anything, because memories of inside the mirror fade once you get out.
Oh that does make the most sense but I don't really remember Horus saying he would get her out of there. I mean the entire mirror situation and which Nephthys is real and which is a fake is still a mystery, especially for Horus.
I assumed it was because she knew he had feelings for Seth? Cuz the nephthys in the mirror is the og nephyths, the one that loves Seth so to me their interaction felt awkward reserved and guarded but I may be wrong
Horus didn't answer when she asked him to promise, that's true (specifically, she had asked him to promise that he'd tell Seth about the *her* trapped inside the mirror, which Horus didn't do either... and yep, the scene got cut short before we could see if he actually said "yes" to that)
I would have to reread it again, but why did Nephthys say she no longer resented Horus? Did he do something to her that I can't remember or does it have to do with him also being Osiris' son just like Anubis is?