I think Drusilla knew Spike was (or would be) in love with Buffy. When she breaks up with him she says 'your surrounded by her' or something like that about Buffy. Dru was psychic and I think she broke up with him because she knew about the feelings he would have for Buffy long before he became aware of them, so if that's right the break up was his fault for falling for another woman, even if he didn't know it yet.

It's his fault because he fell for another woman even though he didn't know it? How is that fair? Dru's the one that did the leaving, ergo she is the one to blame.

I think it's Spikes fault cause he was always a MAJOR JERK 2 Drusilla!

Well it's not Spike's fault he fell for someone else, but it's not Dru's fault either is it? could you stay with someone knowing they loved someone else?

I don't think anyone is to blame as Spike can't help how he feels but Drusilla shouldn't have to stay with someone who loves another woman, but I think Spike's feelings caused the break up. It's not fair, but when is a break up ever fair?
Emotionally Spike did the leaving not Dru. As soon as Spike's feelings switched to someone else he ended the relationship.

spike turned sappy, he wasnt in love with another woman at that point he just didnt wna be proper evil and kept pretending he did and dru didnt want that she wanted her men more like mad and evil and spike was going more and more the opposdite, if he really wanted to keep her he would have been what she wnated!

From eternal love to puppy mush. What more is there to say?

It was her decision. Even though she'd foreseen that he'd love Buffy in future, that hadn't happened yet. Maybe if she didn't leave him or cheat on him, he'd stay with her.