You aren't spanking hard enough, then! That should increase Pain, but not as much as horse-whipping does.
As for getting her to break, I rinsed through the game pretty fast in the first few days of release, before there was any info really about the gameplay mechanics. I made a bunch of girls all with completely different traits and just played.
Looking back through the girls from that playthrough, the one that broke really quickly has the traits Timid, Glass Heart and Sex-Hater. She broke almost instantly and I wasn't even trying, I had no idea the mechanic even existed at that point. I still haven't looked at any of those guides, but looking at all these red bars, looks like Perverted and Masochist and a regime of whipping are a pretty good combo, too.
I think, depending on their traits, if you basically edge them and don't let them cum, that's a quick-ish way to max out their hate, same as cumming in Clean Freaks, I think