Instant disinterest?

Hi there. Okay, so I've been dating my best friend for about a month now, and she's really in love with me. I want to be in love with her, but ever since we started dating, those crush-like feelings have gone away. I now feel nothing but guilt towards her and I kinda want to break it off. But I don't want to break her heart: she's my best friend, and we've been through some crazy shit together. This isn't the first time that I've started dating someone and then instantly lost interest in the relationship. Am I doing something wrong? Should we break up? Do I tell her? What should I do?

tell her the truth and let her decide