Hi everyone,
I haven't read that story and I'm posting for someone else.

Here are the clues:
«Does anybody remember a story where Spike gets his soul (and maybe also becomes human), and ends up training to be a watcher. While he is training, Dawn is experimenting with portals and ends up in his rooms at the...watcher's academy and accidentally leaves behind her necklace. Spike eventually arrives in Sunnydale as Buffy's new watcher, but doesn't intend to renew his relationship with her. In fact, he has a new girlfriend for a while.»

Thanks for your help!

Found it! Ah, I really adore a good fic hunt.

It's The Reset Series by PSUbrat. On EF and on FF.net.
The epilogue of the third story is missing on EF.

Summary: After Tara's death, and Willow's subsequent rampage, a broken Willow says the one phrase that changes everything.