Is it Vice Versa by 1OddDuck? It's incomplete (though still being updated, the last time being 09 Mar 11) at fanfiction . net and the summary is:-
Buffy's life is imploding, and the only one who understands is the one person she's meant to hate. As her life and feelings start to change, Buffy realises Spike may be the only one who can save her. Set in an AU Season 5 without Dawn. B/S.
It does have Buffy and Riley in a rather nice relationship to begin with (boo, hiss!!) but then Buffy starts to feel something toward Spike.
Hope this is the one.

Sarah