I'm not quite saying that. It's not a question of whether she believes in this love. It's that she really is terrified of it. Since Spike is arguably playing the same game, it's a reading I can deal with as a Spike-centric person. If it's just Buffy waiting for Spike to really really really prove his love, then bah, he could do better.

Or to put it another way - until she gets over the fear, she's not going after him, nor would she let him in if he came after her.