Maybe because Buffy wasn't actually invested in the relationship so there was no urgency/need for her to address the issues.
But maybe things changed when she told Spike 'I love you.' It made the whole thing real for her but didn't change how she felt or would ever feel about Spike. That basically she would never love him despite maybe wanting to love him. She thought saying the works would magically make it true becasue why wouldn't it? After so long together.
And as the year rolled on with zero in the way of evil or slaying drama to distract her from the big, bad future coming her way she finally had to come to the realization that it would never work.
Somebody should write a little fan-fic about that, say set right at the end of S11...
It’s a hard sell that she wasn’t invested and although I don’t personally understand it I think the story is pretty clear she loves him. But love isn’t always enough.
But from a fan fic perspective I’m all over your concept!