I could have been less blunt. I know it sucks to be dumped. I don't know your situation better than you do, just trying to provide helpful perspective.
Some people change their genuine love interest quickly, especially if they fall in love easily. Some people take different kinds of emotions more seriously than others. Many of these people are not predatory at all. If she's young, and you're her second partner, maybe she's just emotionally immature.
Don't get me wrong, with "predatory" I didn't mean to indicate some form of conscious behaviour targeting my money, nor do I think that she is faking her current engagement. I guess that in the end we are just giving different names to the same phoenomenon: feeling of love and attraction that adapts very quickly to better circumstances. All in all, it's a very effective survival mechanism for her (and for a fair number of women I happened to know/study), it's just emotionally wrecking for those on the other side... But now that I'm (very painfully) aware of this possibility, I just need to calibrate for that and go on with my life.
Information that surprises you is interesting as it exposes where you have been miscalibrated, and allows you to correct for that.
I suspect the users of LessWrong have fairly similar beliefs, so it is probable that information that has surprised you would surprise others here, so it would be useful for them if you shared them.
Example: In a discussion with a friend recently I realised I had massively miscalibrated on the percentage of the UK population who shared my beliefs on certain subjects, in general the population was far more conservative than I had expected.
In retrospect I was assuming my own personal experience was more representative than it was, even when attempting to correct for that.