I remember this story, which is set in season6 and Spike and Buffy had already started to sleep together, I think, when they found out Buffy was resurrected without a soul and that's why she felt like she came back wrong and why Spike's chip doesn't work with her. I can't remember much else, except that Buffy, feeling like she's free to do whatever she wants now because, what does it matter, she has no soul like Spike, finally allows herself to love Spike and stop feeling guilty about it.

All Except One by EllieRose101

A shock revelation threatens to tear the Scooby Gang apart, but will it bring Spike and Buffy together in the long run? (Alternative Season Seven, without the First Evil and Potential Slayers plot line)

https://dark-solace.org/elysian/modules/challenges/challenges.php?chalid=684

No, it's not this one. Thank you, though

No, it's not this one. Thank you, thoughjanab